Which shell has the capacity to hold only two electrons? 

First shell 

The correct answer is option 1. First shell.

The arrangement of electrons around the nucleus of an atom occurs in electron shells, also known as energy levels. These shells are designated by the letters K, L, M, N, and so on, corresponding to the principal quantum numbers n = 1, 2, 3, 4, and beyond.

The first shell, denoted by the letter K, is closest to the nucleus. In this shell, there is only one subshell, which is an s orbital. According to the Pauli exclusion principle, each orbital can hold a maximum of two electrons, with opposite spins. Therefore, the first shell, being comprised of a single s orbital, has the capacity to hold only two electrons.

This characteristic makes the first shell unique in its electron capacity compared to the subsequent shells, which have more complex configurations with multiple subshells (s, p, d, f) and can hold more electrons. As electrons are added to higher energy levels, they occupy increasingly complex arrangements of orbitals with higher energy. However, the first shell remains limited to only two electrons in its single s orbital.