Answer:
B. 20 ≤ x ≤ 50
Explanation:
If the maximum speed is 50 miles per hour, that means it cannot go above 50 miles per hour, it has to be 50 miles per hour or less, which is expressed like this

Now, if the minimum speed is 20 miles per hour, that means the speed can only be 20 miles per hour or more, that's represented as

If we unit both expression, we would have the following compound inequality

This means that the speed has to be 20 miles per hour or more, but 50 miles per hour or less.
Therefore, the right answer is