What kind of cell results when a diploid and a haploid gamete fuse during fertilization?

The correct answer is: a triploid cell.

Explanation:

A triploid cell is the result of the fertilization of a diploid cell and a haploid cell. Triploid cells have 3 sets of chromosomes, diploids have 2 sets, and haploids have only 1 set.

Humans, besides many other species, have diploid cells - one set of chromosomes comes from the mother and the other set of chromosomes comes from the father. In order to maintain this number, gametes (the sex cells: spermatozoids and oocytes) become haploid through meiosis. This way, when the spermatozoid fertilizes the oocyte, the resulting cell will be diploid.

A gamete being diploid is a result of an error during meiosis, and will most likely generate a triploid cell during fertilization.

A triploid cell, unfortunately, won't be able to survive, so they are almost always spontaneously aborted in the first two weeks of pregnancy.