In question setting terms, this is a schoolboy error.
David Lloyd George was the last Liberal politician to be Prime Minister; but he was not a Liberal Prime Minister in the sense that he led a Liberal government. The last person to do that was his predecessor, H. H. Asquith; Lloyd George only led a Coalition government.
If you want to ask about the last Liberal prime minister (and it's a perfectly reasonable question, if worded unambiguously) you should either ask "Who was the last prime minister to lead a Liberal government?" or "Who was the last Liberal politician to be Prime Minister?" Or words to that effect, in either case.
The way this question was worded, "Asquith" was an equally acceptable answer.
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