Light elements with an atomic number less than 20 generally have a neutron to proton ratio equal to
a.1: 1
b.1.3: 1
c.1.5: 1
d.2: 1

Atomic elements consist of a nucleus that contains protons and neutrons. Protons carry a positive charge whilst neutrons are electrically neutral. In light elements with an atomic number less than 20, the neutron to proton ratio is generally equal to 1:1. This changes for heavier elements since the Coulomb interaction between many protons gets stronger and demands more neutrons for the nucleus to remain stable. 

Ithink answer is cast?