y = sec³(3x)
dy/dx = d (sec 3x)³/dx . d (sec 3x)/dx . d(3x)/dx
y' = dy/dx = 3(sec² 3x) . (tan² 3x) . 3
y' = 9 sec² (3x) tan² (3x)
Hence, option C is correct!
Hope it helps..
