The correct answer is B. When gametes are formed, each gamete carries only one allele for a gene.
Mendel’s the ‘principle of segregation’ or ‘law of purity of gamete’ allowed him to analyse each trait separately. This law is an universal law. According to the principle of segregation, the two alleles of a gene separate or segregate during gamete formation and goes into separate gamete. So, a gamete may have either the dominant allele or the recessive allele but never both the alleles of a gene. Thus, gamete is always pure for a particular trait. 3:1 ratio of offspring in F2 generation of monohybrid cross indicates segregation of factors during gametogenesis and test cross confirms segregation.