The correct answer is: D. examine a gift carefully.
The idiom “look a gift horse in the mouth” means to inspect a gift carefully, especially to look for faults. The phrase is thought to have originated in the Middle Ages, when horses were often traded without being seen by the buyer. In order to avoid being cheated, buyers would often look at the horse’s teeth to assess its age and health.
Today, the phrase “look a gift horse in the mouth” is often used to discourage people from being too critical of gifts they receive. It is a reminder that it is important to be grateful for any gift, regardless of its size or value.
The other options are incorrect because they do not accurately reflect the meaning of the idiom. Option A, “welcome a gift,” is too general. The idiom “look a gift horse in the mouth” is specifically about inspecting a gift carefully, not simply welcoming it. Option B, “find fault with a gift,” is the opposite of the meaning of the idiom. The idiom is about being grateful for a gift, not finding fault with it. Option C, “be fastidious,” means to be very particular or fussy. This is not the same as inspecting a gift carefully.