This is a simple question @Himanshu :)
Given , T(n) = T(n-1) + c
⇒ T(n) = (T(n-2) + c) + c [We substitute for T(n-1) similarly here]
⇒ T(n) = (T(n-3) + c) + c + c = T(n-3) + 3c
.
.
⇒ T(n) = T(n-n) + nc = T(0) + nc
Considering T(0) = some constant , we have
T(n) = nc = O(n)