range capable flyer adjacent to two opposing figures.

One is grounded one is hovering

can he make a ranged attack against both or must he chose the grounded figure

Fliers can ALWAYS make ranged attacks as long as they can draw a clear LOF, no matter who or what is basing them.

So in your above example he can target both figures. The only thing he can't do is target someone who isn't adjacent to him while based by an enemy figure.

And just for the record, both enemy figures in your example are "grounded".

Grounded does NOT mean "not flying". ANY figure who isn't soaring or on elevated terrain is "grounded", hovering or not.