____ said under oath was disputed by several other witnesses ?

The sentence in question involves a clause that specifies the subject of the dispute regarding a statement made under oath. The phrase '____ said under oath was disputed by several other witnesses' requires a word or phrase that can serve as the subject of the statement being referenced.

Option B, "What the man," fits this requirement perfectly because "what" serves as a pronoun that encompasses the entirety of what the man said. Thus, it indicates that the content of the man's statement is the focus of discussion. The complete sentence would read: "What the man said under oath was disputed by several other witnesses," clearly indicating that the subject of the dispute is the man's statement itself.

In contrast, the other options do not fit well in this context. Option A, "It is the man," is grammatically incorrect; it introduces an incomplete thought. Option C, "That the man," lacks a preceding context that would allow it to stand alone as a complete clause. Finally, option D, "The man," does not reference the content of the statement but merely identifies the individual, making it insufficient for the intended meaning. Thus, option B is the most coherent and grammatically correct choice.