Answer
See explanation below.
Work Step by Step
Cosmic rays contain high energy neutrons which keep the 14C constant by entering into upper layers of atmosphere , by bombarding ordinary 14N atoms.
Living organisms absorb the 14C and hence its proportion remains constant.
This is given by the following reaction:
14 N 7 + 1 n 0 --> 14 C 6 + 1 H 1