This can be understood by drawing on a graph. However it can also be done by hit-and-trial/option elimination :
a)If p lies between infinity and 0, suppose p is -10, then p-1=-11. Product is greater than 0
b)p lies between 0 and 1 , suppose p is 0.9 and p-1= -(.1) Less than 0
c) Let p=10 then p-1=9 product is greater than 0
d) Let p=8, p-1=7 Product is greater than 0