We know that atomic mass is numerically equal to mass number of an atom. Since the helium atom has an atomic mass of 4u, therefore, the mass number of helium atom will be 4. And the number of protons in the helium nucleus has been given to be 2. Now,

Mass number = No. of protons + No. of neutrons

4 = 2 + No. of neutrons

And, No. of neutrons = 4 – 2 = 2

Thus, the helium has 2 neutrons.