Read the passage below and answer the question.

Atom contains dense mass at its center, called nucleus. Nucleus is positively charged. Nucleus is made up of neutrons and protons. Therefore, it may be expected that the mass of the nucleus is equal to its individual protons and neutrons. However, the nuclear mass M is found to be always less than this. 

What fits the best in this?

If one wants to break the nucleus into its respective neutrons and protons, an extra amount of energy equivalent to the difference in the masses discussed above must _____ .

be supplied

As mass of constituents individually is greater than the nucleus as a whole, the extra energy must be supplied to break the nucleus into its constituents.