The answer is D. The cow stood in udder amazement when she was unable to produce milk.
The reason is that puns usually use a play of words to make the statement sound funny or ironic. The word may have a double meaning, like in this example. For example, they replaced the word "utter" with "udder", which sound similar.
Answer:
The answer is D. The cow stood in udder amazement when she was unable to produce milk.