(c)
Given, AP is $3,\ a_{1},a_{2},a_{3},a_{4},a_{5},a_{6},31$
$\therefore\ 31 = 3 + 7d$
$\Rightarrow d = 4$
$\therefore\ a_{1} = 3 + 4 = 7$
$a_{5} = a + 5d = 3 + 20 = 23$
and $a_{6} = a + 6d = 3 + 24 = 27$
$\therefore\ a_{6} - a_{5} = 27 - 23 = 4$
and $a_{1} + a_{6} = 7 + 27 = 34$