Billy often eat hamburgers, but he did not always have then with cheese. Billy are cheeseburgers more often than some people, but less often than others. You could say that it was something he did only when?\nOptions:\n- occasionally\n- seldom\n- hardly ever\n- infrequently\n- never
A step-by-step solution is:
occasionally
Occasinally is a time where people do it more often than some but yet less often than other.

Occasionally means at irregular intervals which is how billy ate hamburgers.

.

So, the final answer is occasionally