Metonymy : 'crown' is a metonym, used to represent the reigning monarch (king or queen, depending who is on the throne).
A metonym is a word, name, or expression used as a substitute for something else with which it is closely associated. 'crown' for the monarch/ the monarchy; " Washington" as a metonym for the US government.
(Also see : synecdoche ( SIN - EK - Də - KEY )
This is a figure of speech in which a PART is made to represent the whole or vice versa, as in :
"England lost by six wickets." (meaning ‘ the English cricket team ’).
HOWEVER - be aware that both metonymy and synecdoche are subsumed under the generic term METAPHOR, so the proper answer might be:
It is a metaphor; specifically, an example of metonymy.
