Nescio quid mater mea faciat.
The form faciat is used in this sentence because it is the verb in...
- a purpose clause.
- an indirect question. (correct answer)
- a result clause.
- a simple statement of fact.
Explanation: The clause quid mater mea faciat is an indirect question, dependent on the main verb nescio ("I do not know"). Indirect questions are introduced by an interrogative word (here, quid) and always have their verb in the subjunctive mood. A statement of fact (D) would use the indicative mood. Purpose (A) and result (C) clauses use the subjunctive but are introduced by conjunctions like ut or ne.