Answer:
Option B, There are 48 neutrons within each isotope
Step-by-step explanation:
As we know , 
Mass number (M) is equal the sum of atomic number (A) and neutrons (n)
Mathematically, it can be represented as - 

Given, 
Atomic Mass (M) 

Atomic Number (M) 

Substituting the given values in above equation, we get - 

Option B is correct