For light elements, what is the ratio of neutrons to protons that predicts a stable nucleas?
10: 5
2: 1
1: 1
1.5: 1

Answer is: the ratio of neutrons to protons is 1:1.

For example, nitrogen atom (N-14) has 7 protons (p⁺) and 7 neutrons (n°). The neutron–proton ratio (N/Z ratio or nuclear ratio) is 7 : 7 ( 1 : 1).

Another example, calcium atom (Ca-40) has 20 protons (p⁺) and 20 neutrons (n°). The neutron–proton ratio is 20 : 20 ( 1 : 1).

The neutron–proton ratio generally increases with increasing atomic number.