The choices on pizza are plain, sausage, or pepperoni. Thus, there are three types of pizza and a guest can choose only one type.
Likewise, the choices on drinks are milk, orange juice, apple juice, or lemonade. Thus, there are four types of drinks and a guest can choose only one type.
This is a typical case of Combination where for pizza the guest has to choose 1 from 3 and for drinks the guest has to choose 1 from 4. Thus, we have the following setup:
For Pizza:
For Drinks: 
Thus, the total number of combinations possible will be:


Therefore, 12 is the answer.