Basically: Whenever you have a number times a variable, you don't have to write the multiplication sign.
5 × n is the same thing as 5n.
Let's write each of these out.
a) 5 + 2 + x
b) 5 + 2x
c) 5 + x + x
d) 5 + x² (the ² means multiply by itself)
e) 5 + 2x (sum = adding together)
f) 5 × 2x (product = multiplying together)
As you can see, only B and E fit the expression given.