During DNA replication occasional errors can occur resulting in a change in a nucleotide base, for example instead of adding a T, a C is added. If this is not corrected by mismatch repair or other mechanisms, this change represents a

MUTATION

Explanation:

As said in the question, changes in nucleotide bases that make up the genetic material (DNA) occur from time to time. This change called MUTATION can either occur as a result of copying mistakes during replication or be induced by mutagens (mutation-causing components). If the affected cell is not internally corrected using mismatch pair mechanism and it keeps dividing, it could possibly serious problems to the affected organism.

The kind of change described in the question is a SUBSTITUTION MUTATION, in which a nuceleotide base replaces another in the DNA sequence i.e. C replaces T.

the correct answer is 2 and 4