Next weekend, I . . . . . . . . my proficiency exam, so I . . . . . . . . more or less 300 questions so far.

Answer: Option B

The best answer is "am having / will have finished" because the sentence refers to a scheduled future event and an action that will be completed before that future event.

"I am having my proficiency exam" uses the Present Continuous Tense, which is commonly used to express fixed future arrangements or scheduled events such as exams, meetings, and appointments.

"I will have finished" is the Future Perfect Tense, which indicates that the action of solving approximately 300 questions will be completed before the proficiency exam next weekend.

Although the phrase "so far" is normally associated with the Present Perfect Tense, its use in this question makes the sentence slightly awkward. In standard English, "by then" would be more appropriate with the Future Perfect: "I will have finished more or less 300 questions by then." Despite this wording issue, Option B is the only option that maintains the correct sequence of tenses for a future reference point.

Examples:

By next Friday, I will have completed the assignment.

I am having my final interview next Monday.