One of the most important problems they had to consider was________of public health. A. what B. this

One of the most important problems they had to consider was________of public health. A. what B. this C. that D. which

The correct choice is C. that.

The sentence structure indicates a need for a word that can stand in for a concept or issue already mentioned or implied. Here, "that" functions as a demonstrative pronoun referring back to "one of the most important problems."

Let's break it down:

The sentence is: "One of the most important problems they had to consider was ________ of public health."

The blank needs a word that connects "the problem" to the specific domain "of public health."

"The problem of public health" is a standard noun phrase. To refer to this idea abstractly in the sentence structure, we use the pronoun "that."

Why the others are incorrect:

A. what: This would be used to introduce a clause (e.g., "what to do about public health" or "what the main issue of public health was"). It doesn't work alone before "of public health."

B. this: While also a demonstrative pronoun, "this" is typically used for something more immediate, specific, or physically/mentally present. "That" is more appropriate for referencing a previously stated or more abstract concept.

D. which: This is a relative pronoun used to introduce a relative clause (e.g., "which was a problem of public health"). It cannot stand alone as the object of the verb "was" in this way.

Therefore, the complete and correct sentence is:

One of the most important problems they had to consider was that of public health.

This means: "They had to consider the problem of public health, which was one of the most important problems."