Answer : (c) 8

Explanation : Let the four terms be $\;a-3d\;,a-d\;,a+d\;,and\;a+3d$ with common difference 2d.

$sum=4a=20$

$a=5$

$\frac{(a-3d)(a+3d)}{(a-d)(a+d)}=\frac{2}{3}$

$\frac{25-9d^2}{25-d^2}=\frac{2}{3}$

$d^2=1$

$d=1$

$Largest \;term=a+3d$

$=5+3(1)$

$=8.$