Let the total amount of oxide be 100

$\therefore$ mass of oxygen is 20

$\therefore$ mass of element is (100-20)=80

$\therefore$ equivalent mass of element=$\large\frac{mass\;of\;element}{mass\;of\;oxygen}$$\times 8$

$\qquad\qquad\qquad\qquad\qquad=\large\frac{80}{20}$$\times 8$

$\qquad\qquad\qquad\qquad\qquad=32$

Hence (b) is the correct option.