Which process occurred frequently in the early history of the three domains on earth, and makes determining phylogenetic relationships of that time difficult?

a. horizontal gene transfer
b. alternative rna splicing
c. meiosis
d. binary fission
e. mitosis

The correct answer is - option A. horizontal gene transfer.

Explanation:

Horizontal gene transfer allows the gene movement or the flow from the genome of the one organism to another genome of the organism. It occurs in between prokaryotes of different types of species commonly even between prokaryotes and eukaryotes.  

Thus, the correct answer is - Option A. horizontal gene transfer.