The operation $\Delta$ is defined so that $a \Delta b=a \times b+a+b$. For example, $2 \Delta 5=2 \times 5+2+5=17$. If $p \Delta 3=39$, the value of $p$ is
(A) 13
(B) 12
(C) 9
(D) 10.5
(E) 18
Using the definition of $\Delta$, we see that $p \Delta 3=p \times 3+p+3=3 p+p+3=4 p+3$.

Since $p \Delta 3=39$, then $4 p+3=39$ or $4 p=39-3=36$ and so $p=\frac{36}{4}=9$.

ANswer: (C)