Capella is a binary star system consisting of two stars, but those two stars are each a part of a larger system with additional stars. Specifically, Capella has two primary stars (Capella A and Capella B) and is part of a wider system that includes two more fainter stars, making it effectively a group of four stars. So, while the primary answer is 2, it can also be recognized as having a total of 4 stars in its system.
Given your options, the most straightforward answer would be 2, but in context, it could also be considered 4 due to the additional stars in the system. Choose your answer based on the context of your question!