When a single gene locus has more than two possible alleles, this is called ?

Polyallelism. 
Poly means many. So, when there are more than two alleles for the same gene locus we call it of polyallelism. This makes genetic variability much bigger because there are several alleles for a possible location in a chromosome and this would make, for example, from two parents a possible combination of two out of four possible alleles if between them they had all different alleles.

C. a mature ovule of a plant.