Which is a frameshift mutation? a. loss of one codon b. a 3-base insertion c. a 6-base deletion d. a 2-base deletion e. a 2 codon insertion?

Option d, a 2-base deletion

Explanation:

In a frame shift mutation, number of base (in a multiple of three) gets deleted. When a single nucleotide is lost, the gene loses its important function.

However, when nucleotide bases are lost in pair of three (as codon) or in some number which are multiple of three such as 6, 9, 12 etc. a gene still has the ability to restore is vital function.  

Hence, option d is correct.