de Vries believed that mutation caused speciation and called it as -

The correct answer is Option (3) → Saltation

Hugo de Vries worked on the evening primrose. He proposed that evolution is caused by mutation, not by the small variations Darwin talked about. For Darwin evolution was gradual, but de Vries believed that mutation caused new species to form in a single step. He called this saltation, a single step large mutation.

(1) Gene pool — Incorrect. The gene pool is the total of all the genes and their alleles in a population.

(2) Gene flow — Incorrect. Gene flow is the movement of genes from one population to another through migration. It is one of the factors that disturb the Hardy-Weinberg equilibrium.

(3) Saltation — Correct. This is the term de Vries used for speciation by a single large mutation.

(4) Natural selection — Incorrect. Natural selection is Darwin’s mechanism. It works on small variations and leads to gradual evolution.

NCERT Extract:
Mechanism of Evolution – “… In the first decade of twentieth century, Hugo deVries based on his work on evening primrose brought forth the idea of mutations – large difference arising suddenly in a population. He believed that it is mutation which causes evolution and not the minor variations (heritable) that Darwin talked about. Mutations are random and directionless while Darwinian variations are small and directional. Evolution for Darwin was gradual while deVries believed mutation caused speciation and hence called it saltation (single step large mutation).”