I´d say A. Others don´t make much sense in the context.
Explanation:
Astronomers have identified more spiral galaxies than ellipticals, but that's simply because the spirals are easier to spot. While spiral galaxies are bright, elliptical galaxies are dim. The existing stars inside an elliptical galaxy tend to be older, giving off more red light than younger stars.
The irony in To Kill a Mocking Bird is verbal, dramatic, and situational irony.
B seems to be the most reasonable
