Outside a nucleus: A. Neutron is stable. B. Proton and neutron both are stable. C. Neutrons are unstable. D. None of the above.
Hint: As a first step, one could read the question well along with the given options. One could then recall all the properties of protons as well as neutrons. One could then check for the point related to the stability of these atomic particles outside of the nucleus among them and hence answer the question.
Complete step-by-step solution: In the question, we are given a phrase and we are supposed to complete it into a sentence from the given options. We are actually required to find the true condition for protons and neutrons outside a nucleus. Let us recall the properties of neutrons and protons prior to directly going into the solution. A neutron is one of the subatomic particles that is known to have no net electrostatic charge and is also known to have mass that is approximately equal to that of the proton. When neutrons are kept isolated in space, that is, outside of a nucleus, they decay with a half life of 15 minutes resulting in a proton, an electron and an antineutrino. You may be familiar with this process which is beta decay. By virtue of this process, we could conclude that the space is relatively devoid of free neutrons. Protons are another sub atomic particle that constitute atomic nuclei with positive charge. However, protons can be considered stable outside of nuclei. Therefore, we found that, ‘Outside a nucleus neutron is unstable.’ Hence, option C is correct.
Note: Free protons are known to be routinely used for proton therapy and other particle physics experiments, for example, the large hadron collider. Every atomic nucleus except for hydrogen consists of neutrons. Isotopes are the elements with the same number of protons but different number of neutrons.