Fill in the blank with correct Adjective clause: The club chairman, _…

The correct answer is Option (3) → whom

Complete sentence: The club chairman, whom the member thought to be trustworthy, has just disappeared with the club's money. The clause stands for "the member thought him to be trustworthy". The pronoun is the object of "thought", so the object form is needed.

Option	Form and use	Example
(1) by whom	"By whom" would make the chairman the doer of an action in the clause, but "the member thought by whom to be trustworthy" makes no sense.	–
(2) whose	Whose shows possession and must be followed by a noun (the chairman whose car was stolen). No noun follows here.	–
(3) whom (answer)	An adjective clause describes a noun and begins with a relative pronoun. The pronoun takes the form that fits its job inside the clause: who when it is the subject of a verb, whom when it is the object of a verb or a preposition.	The boy whom we believed to be honest took the money.
(4) who	Who is the subject form. It would fit "who the member thought was trustworthy", where it is the subject of "was". With "thought ... to be", the pronoun is the object, so "whom" is required.	–