What is a holotype?

A holotype is a single physical specimen that serves as the standard for a newly described species. It is the physical example that is preserved in a museum or other collection for future reference and serves as the standard against which all other specimens of that species are compared. The holotype is regarded as the best representative of the species and serves as the basis for the species' description in the scientific literature.

The other options are incorrect definitions of a holotype. 

"A currently formed and described fossil species" is not a holotype definition. 

"A term used to explain the special type of bones found only in birds" is unrelated to the term holotype. 

"The first fossil of a prehistoric animal discovered always becomes the holotype" is also incorrect, as the first fossil discovered may not be the best representative of the species and may not be designated as the holotype.