Positrons are spontaneously emitted from the nuclei of
(1) potassium-37 (3) nitrogen-16
(2) radium-226 (4) thorium-232

Option (1) is the correct answer.

Explanation:

A positron is a small particle which contains a +1 charge and its mass is equal to the mass of an electron, that is, kg.

A positron is represent by the symbol .

For example,

Whereas nitrogen-16 does not decay to give a positron because nitrogen-16 is the daughter isotope formed due to decay of oxygen-16.

The decay reaction is as follows.

           

And, on radioactive decay of thorium-232 there will be formation of radium-228 along with emission of an alpha particle.

The reaction will be as follows.

           

Therefore, we can conclude that out of the given options positrons are spontaneously emitted from the nuclei of potassium-37.