When you use Tsareena's "Queenly Majesty" Ability, when the opponent reveals their hand do you get to choose which card to discard or is it random?
You get to choose.
Source: Sun & Moon FAQ (2017-02-02), TPCi Rules Team (2017-02-02)

Relevant cards

Note: All applicable cards may not be listed below.
SM1_20
SMP_SM26
SMP_SM26