The average isotopic mass or atomic mass = $\sum Mi \times \large\frac{xi}{100}$

Where $m_i$= mass of i th isotope

$x_i$= abundance of ith isotope.

$\therefore $ Atomic mass $=54 \times \large\frac{5}{100}$$+56 \times \large\frac{90}{100}$$+\frac{5}{100}$

$\qquad= 55.95$

Hence b is the correct answer.