What is the fewest number of base changes in a gene that can result in a complete loss of biological activity in the protein that gene codes for?

The correct answer is- 1

Explanation:

A change in only a single base pair of a gene can be responsible for complete loss of biological activity of protein that gene codes for because a change in single base pair can cause a point mutation.  

A change in single base pair can change the reading frame of a codon like in substitution reaction, transition or transversion. A changed nucleotide alters the codon sequence which then can code for different protein or non-functional protein which results in loss of biological activity of that protein.

Therefore the correct answer is only a single number of base change can result in complete loss of biological activity.