Which of the following reactions is an example of nucleophilic substitution reaction ?

Question

( A ) $2RX+2Na\rightarrow R-R+2NaX$
( B ) $RX+Mg\rightarrow RMgX$
( C ) $RX+H_2\rightarrow RH+HX$
( D ) $RX+KOH\rightarrow ROH+KX$