what is trinomial nomenclature?

Nomenclature is a science of providing distinct and proper names to organism so that they can be easily recognized and differentiated from others. 

In trinomial nomenclature, the names of taxa have three parts. The first part is the name of genus, the second part is the name of species and third part is the name of subspecies. For example, <i>Brassica oleracea botrytis</i>.

So, the correct answer is 'Subspecies'.

In biology, trinomial nomenclature is used to name living things at a rank lower than species. The term trinomial nomenclature means "three-part name" or "system of three-part names". If living things r than species get a scientific name, each name has three parts. Binomial Nomenclature: Binomial nomenclature was given by carolus linnaecus in his tenth edition of system natura published in 1758. Binomial nomenclature is defined as system of nomenclature that denote species of living organism.