Answer
$72$
Work Step by Step
The fundamental counting principle says that if there are $|A$ outcomes of the event $A$ and $|B|$ of the event $|B|$, then the number of outcomes of event A and event B combined is $|A|\cdot|B|$. This can easily be extended to $n$ events.
Using Fundamental Principle of Counting the number of one-topping pizzas that can be ordered is: $3\cdot4\cdot6=72$
