C1: $16^{4}$ : $4^{2}$
= ( $4^{2}$ )$^{4}$ : $4^{2}$
= $4^{8}$ : $4^{2}$
= $4^{6}$
C2: $16^{4}$ : $4^{2}$
= $16^{4}$ : 16
= $x^{2}$
= $16^{3}$ 
= $4^{6}$
C1: $25^{2}$ : $5^{2}$
= ( $5^{2}$ ) $^{2}$ : $5^{2}$
= $5^{4}$ : $5^{2}$
= $5^{2}$
C2: $25^{2}$ : $5^{2}$
$25^{2}$ : 25
= 25
= $5^{2}$ 
Chúc bạn học tốt!