Select the type of wordy expression that best describes the sentence below.
They conducted an investigation.
O Modifier problem
Dummy subject
Smothered verb
Redundant expression

C, Smothered verb

Explanation:

Process of elimination:

A: A modifier problem is where a word, phrase, or clause is misplaced in a sentence. The words are in the correct order in "They conducted an investigation." (e.g. when trying to describe that a potter sold all but some of his pottery, the sentence may be "The potter almost sold all of his pottery," when it should be "The potter sold almost all of his pottery,").

B: A dummy subject is something that does not specify what is doing a certain action in a sentence (e.g. "It's cold outside," What's cold outside?).

D: A redundant expression (more commonly known as a tautology) is restating the same thing in two different words (e.g. a sentence may be "during this period of time" where it should be "during this period" or "during this time").

A smothered verb is a noun that could have been a verb. In this example, the sentence is "They conducted an investigation," where the sentence should be "They investigated,".

the answer is irony!