Option (C) 0
For an AP, the common difference is constant: $(p+1)-(p-1) = (2p+3)-(p+1)$
$\Rightarrow 2 = p+2 \Rightarrow p = 0$
Use the AP condition: middle term – first term = third term – middle term, i.e., $2b = a + c$. Set the two consecutive differences equal and solve. A common error is arithmetic mistakes while simplifying — write each step clearly.