Number of possible ways to select 1 type of spice = 3c1 = 3
Number of possible ways to select 2 types of cheese = 4c2 = 6
(Here we can pick our answer as D- 450 since it is the only answer that is divisible by 3*6= 18 , but most likely on the GMAT there will be more another answer that will be divisible by 18 )
Number of possible ways to select 4 types of meat but without pepperoni and anchovies together
= Total number of ways without any restriction - Total number of ways in which pepperoni and anchovies together
= 7c4 - 5c2
=35 - 10
=25

Possible ways to garnish the pizza are available to the customer = 3*6 * 25
=450

Answer D
_________________

You need to select 4 meat out of 7. So if you consider pepperoni and anchovies together , 2 out 4 slot is filled. Remaining 2 slot can filled with remaining 5 options.
Therefore , 5c2