Which species is the reducing agent in the reaction: 3ca (s) + 2cr3+ (aq) 2cr (s) + 3ca2+ (aq)?

The redox reaction is as follows
3Ca + 2Cr³⁺ --> 3Ca²⁺ + 2Cr
in the oxidation reaction the species donates electrons and becomes oxidised.
the oxidised species donates electrons to the other species and reduces the other species hence its called the reducing agent as it reduces the other species. the oxidised species' oxidation number increases.
in the reduction reaction, the species takes in electrons and becomes electrons and its called the oxidising agent as it takes in electrons from the oxidised species thus oxidising the other species. the reduced species' oxidation number decreases.

in this case , Ca becomes oxidised
Ca --> Ca²⁺ + 2e
oxidation number of Ca increases from 0 to +2. since it reduces Cr from Cr³⁺ to Cr, Ca is called the reducing agent

hence answer is Ca, its the reducing agent