Look at the reaction below.

Upper H subscript 2 upper S upper O subscript 4 (a q) plus upper Upper M g (s) right arrow Uper M g upper S upper O subscript 4 (a q) plus upper H subscript 2 (g).

Which substance is the acid in the reaction?
Mg(s)
H2(g)
MgSO4(aq)
H2SO4(aq)