In which phenomena does one gene pair hide the effect of other unit?
The definition of "gene", according to The American Heritage Science Dictionary is:

"A segment of DNA, occupying a specific place on a chromosome, that is the basic unit of heredity. Genes act by directing the production of RNA, which determines the synthesis of proteins that make up living matter and are the catalysts of all cellular processes. The proteins that are determined by genetic DNA result in specific physical traits, such as the shape of a plant leaf, the coloration of an animal's coat, or the texture of a person's hair. Different forms of genes, called alleles, determine how these traits are expressed in a given individual. Humans are thought to have about 35,000 genes, while bacteria have between 500 and 6,000. See also dominant, recessive. See Note at Mendel."

So, when the question refers to genes, it must be referring to two different genes on two different loci. If the question was referring to alleles of the same gene on the same locus, the correct answer would be dominance, but since it is referring to two different genes, the answer is epistasis.