Which of the following tend to react by gaining 1 electron

Option (c) is the correct answer. Halogens react by gaining one electron.

The halogens in the periodic table are Florine, chlorine, Bromine, iodine, astatine

All these electrons are short of one electron in the outermost shell. so these halogens gain one electrons and form anions.

For example, Florine has an atomic number of 9. so its electronic configuration is given by 2,7. it needs one electron to complete its octet as

F + e> F⁻

All other halogens similarly gain one electron to react.

d) the number of neutrons plus the number of protons