There's no single "most common" question on the PSAT. The PSAT, like the SAT, aims for a diverse range of questions testing different skills and concepts within Reading, Writing and Language, and Math. The test makers intentionally vary the questions from year to year to prevent memorization and ensure fair assessment.
While specific question *types* are more frequent than others (e.g., sentence completion in Reading, data interpretation in Math), pinning down one single question as the most common is impossible and misleading.