Here are the answers to your questions, choosing the sentence closest in meaning to the root sentence:

Question 46. People should exercise regularly to stay healthy. A. People have to exercise regularly to stay healthy. Explanation: "Should" and "have to" both express obligation or necessity, although "have to" can be stronger. "Ought to" is also similar to "should" but less common. "Mustn't" expresses prohibition, and "could" expresses possibility.

Question 47. Perhaps Nga knows the answer. B. Nga may know the answer. Explanation: "Perhaps," "could," and "may" all express possibility. "Would" expresses a hypothetical situation, and "should" expresses obligation or expectation. "May" is the most direct and common modal verb to express possibility similar to "perhaps."

Question 48. The best thing for Nick to do is calm down. C. Nick must calm down. Explanation: Saying something is "the best thing to do" implies a strong necessity or obligation. "Must" and "has to" both express strong obligation. "Would" expresses a hypothetical, and "shouldn't" expresses prohibition.

Question 49. Phong wants a cup of coffee. B. Phong would like a cup of coffee. Explanation: "Wants" and "would like" are both used to express a desire. "Would have" suggests a conditional situation, "should have" suggests advice, and "likes enjoying" is grammatically correct but not the closest in meaning to the simple statement of wanting.

Question 50. It's dangerous to wash your hair when you have a headache. A. You shouldn't wash your hair when you have a headache. Explanation: Saying something is "dangerous" implies that it is not advisable or recommended, thus "shouldn't" (should not) is the closest in meaning. "Will" expresses a future action, "should" expresses recommendation (the opposite of dangerous in this context), and "mush" is not a correct modal verb; "must" would imply necessity, which is not the meaning here.