Modals: They are verbs which are used before the main verb to express ability, possibility, obligation, permission, suggestion, request, certainty or necessity.They indicate the attitude, intention and the mood of the speaker.

• They cannot be used alone in a sentence.
• In case of Questions they are used alone.
• They are not affected by the change in person i.e. first, second or third form.
• Present Participle or Past Participle form of a verb is never used with a modal.

Use of Should:

1. Should can be used to express duty, responsibility, advice, probability, expectation, presumption, moral obligation and mild command e.g.

   (a) One should obey ones elders. (duty)

2. Should is also used for expressing purpose like may and might. e.g.

       I taught her so that she should pass. (she might pass)

In the question : -

1.When you go to London, you __________ go to the theatre.

In this sentence , should is the correct modal that can be placed in the blank part as it is used to give opinion or advice .

2. He ______________  gone by morning bus .

In this sentence , should have is the correct modal that can be placed in the blank part as it is used in perfect tense denotes an obligation that was not fulfilled. 

Thus , the Correct Option is D , which is should , should have .

Modals: They are verbs which are used before the main verb to express ability, possibility, obligation, permission, suggestion, request, certainty or necessity.They indicate the attitude, intention and the mood of the speaker.

• They cannot be used alone in a sentence.
• In case of Questions they are used alone.
• They are not affected by the change in person i.e. first, second or third form.
• Present Participle or Past Participle form of a verb is never used with a modal.

Use of Should:

1. Should can be used to express duty, responsibility, advice, probability, expectation, presumption, moral obligation and mild command e.g.

   (a) One should obey ones elders. (duty)

2. Should is also used for expressing purpose like may and might. e.g.

In the question : -

1. You ________ find her in the playground.

In this sentence , should is the correct modal that can be placed in the blank part as it is used to show probability or possibility of something .

2. There ________ be a law against that.

In this sentence , should is the correct modal that can be placed in the blank part as it is used to offering advice or opinions (similar to ought to) .

Thus , the Correct Option is C , which is should , should .

Modals: They are verbs which are used before the main verb to express ability, possibility, obligation, permission, suggestion, request, certainty or necessity.They indicate the attitude, intention and the mood of the speaker.

• They cannot be used alone in a sentence.
• In case of Questions they are used alone.
• They are not affected by the change in person i.e. first, second or third form.
• Present Participle or Past Participle form of a verb is never used with a modal.

Use of Should:

1. Should can be used to express duty, responsibility, advice, probability, expectation, presumption, moral obligation and mild command e.g.

   (a) One should obey ones elders. (duty)

2. Should is also used for expressing purpose like may and might. e.g.

In the question : -

1. This _________ be Mark’s house.

In this sentence , shouldn't is the correct modal that can be placed in the blank part as it is used to show probability / expectation .

2. You __________ go to the hospital tomorrow or you will be more sick.

In this sentence , should is the correct modal that can be placed in the blank part as it is used to show advice .

3. For going abroad, you ___________ go to a language course first, I am sure it will be very useful.

In this sentence , should is the correct modal that can be placed in the blank part as it is used to show advice or suggestion .

Thus , the Correct Option is C , which is shouldn't , should , should .