A collection of all the alleles of all the genes of a crop plant is called:

A germplasm collection is the collection of all the variety of alleles for all genes in a given crop.

Collection of protoplast is known as protoplasm collection.

A herbarium is a collection of dried leaves of plants for the purpose of identification.

Somaclonal collection is the collection of clones formed by the process of somatic hybridisation.

Hence (a) is the correct answer.