Bismuth is the largest stable, naturally occurring isotope. Which of the following is the reason for this limitation?

I don't see the options in the question, anyway we can answer it.

Bismuth (Z=83) is the largest stable isotope existing in nature. The main reason for that is that elements with Z>83 have too many nucleons in their nucleus. When a nucleus has too many nucleus, it tends to decay (for example, via alpha decay, emitting 2 protons and 2 neutrons), in order to become more stable.