Choose the option that best completes the sentence. If her brother ____ here, he would call the police.

The correct option to complete the sentence is "were". Let's understand why:

This sentence is an example of a conditional sentence. Specifically, it is a type called the second conditional, which is used to talk about hypothetical situations that are not real or are unlikely to happen in the present or future.

The structure of a second conditional sentence is: If + subject + past simple, subject + would + base verb. Here is the structure applied to the sentence:

• If her brother were here, he would call the police.

In the conditional clause "If her brother were here," we use the past simple verb "were" which is used for hypothetical situations involving the subject "her brother."

Note: Although "were" typically pairs with plural subjects, in the case of hypothetical or conditional sentences, "were" is also used with singular subjects like "her brother." This use is sometimes referred to as the subjunctive mood.