Select the most suitable option to fill in the blank:

Some of you are learning Russian ________.

aren't you?

The correct answer is Option (4) - aren't you?

Reasoning:
This question tag must agree with the auxiliary verb and subject in the main statement.

Sentence:

Some of you are learning Russian, ________?

The main verb is “are learning”, and the subject is “you” (from “some of you”).
When forming a question tag, we use the opposite polarity — since the main clause is affirmative, the tag must be negative.

 Therefore,
→ Some of you are learning Russian, aren’t you?