Answer: D. Half.
Explanation: In english, most of the singular nouns, form their plural by adding "-s" at the end of the word, but there are exceptions and irregular plurals. From the given options, the plural of the noun "bluff" is "bluff" too, wildlife is an uncountable noun, so it doesn't have a plural form. The plural of knockoff is regular, so it is "knockoffs" and finally the one that requires dropping the last letter and adding ves to make it plural is "half" which plural form is "halves."