Answer:
b. allele
Step-by-step explanation:
An allele is a variant form of a gene. In this problem, there is one gene (that determines seed color) with two possible alleles (green and yellow).
The other options are wrong because:
The genotype is the combination of alleles an individual has.
The karyotype is an individual's collection of chromosomes, paired and ordered.
A homozygote individual has the same alleles for a particular gene.
A heterozygote individual has different alleles for a particular gene.