Question

You can borrow my tennis racquet, but you _____ keep it very well. It’s very expensive. Choose the correct word to fill in the blank.
A. don't have to
B. lest
C. must
D. could

Answer 1

Hint: In linguistics and philosophy, the phenomenon by which a language can be used to discuss possible situations is known as a modality. For instance, a modal expression (indicating modality) may convey that something is likely, desirable, or permissible. Therefore, the modals are those words which can be used to indicate modality, that is, likelihood, ability, permission, request, capacity, suggestions, order, obligation, or advice.
They can be noticed in sentences where they are accompanying the base form of another verb having content that is relating to meaning in language.

Complete answer:
1. Here we are going to check each option one by one so that we can detect the correct option containing the proper form of the verb.
2. If we look at the first option- “don’t have to”, we can see that it is the negative form of ‘have to’. It is used in sentences where we want to remove necessity or external obligation. But if it is used in this sentence, it would eliminate the necessity or obligation to keep the racquet safe, which is contrary to what the speaker is trying to convey. Also 'don't have to' is used with negative sentences but this is a positive one. Hence, this option is not correct.
3. ‘Lest’ is used with the intention of preventing (something undesirable). It cannot be used in this sentence because it will not fit in correctly (will oppose the rules of grammar). Hence, this option is incorrect.
4. ‘Must’ is used when we try to express necessity or compulsion. This is the single option which complies with the situation.
5. ‘Could’ is used to express ability or lack of ability depending on the situation where it is being used. It is not suitable for this sentence as the speaker is trying to prove his necessity not a suggestion or a request like- ‘Could you keep this?’

Hence the correct answer is option ‘C’.

Note:Here in the question, it is clearly evident that the speaker is expressing that the other person can borrow the racquet but he necessarily should keep it safe and nicely so that it is not harmed, because it is very expensive. Here we can see that as per the syntax of the sentence, we have to add the word ‘must’. This adds the sense of compulsion which we can assume from the other given words in the sentence.