which
pron.pron. used to ask about or point to a specific person or thing from a small group. You use it when there are limited choices available.
pron. an interrogative or relative pronoun used to refer to one or more people or things from a definite set. Often introduces a non-restrictive relative clause when following a comma.
Which color do you prefer, blue or red?
The manager asked which of the candidates had the most experience with digital marketing tools.
The report, which was published late last night, details several significant errors in the previous year's financial projections that had gone unnoticed by the board.
From Middle English which, hwic, wilche, hwilch, whilk, hwilc, from Old English hwelċ (“which”), from Proto-Germanic hwilīkaz (“what kind”, literally “like what”), derived from hwaz. By surface analysis, who + like. Cognates include Scots whilk (“which”), West Frisian hokker (“which”), Dutch welk (“which”), Low German welk (“which”), German welcher (“which”), Danish hvilken (“which”), Swedish vilken (“which”), Norwegian hvilken (“which”), Icelandic hvílíkur (“which”).
As a relative pronoun, it typically introduces non-defining clauses and is preceded by a comma; 'that' is preferred for defining clauses in many style guides.
The car who I bought is fast.The car which I bought is fast.Use 'which' or 'that' for objects and animals; 'who' is reserved for people.