Ok, let's write all the possible selections and the number of options for each selection:
1) Toppings: Here we have 10 options.
2) Crusts: 9 options.
3) Cheese: 6 options.
Now, the total number of different combinations is equal to the product of the number of options for each selection.
So we have:
10*9*6 = 540 different possible single-topping pizzas.
the answer to this question is in the pic