Triangle A B C is shown. Angle B C A is a right angle. The length of hypotenuse A B is 5, the length of B C is 3, and the length of A C is 4. What is the length of the side opposite Angle B? 3 units 4 units 5 units 6 units

The side opposite to angle B is the side that does not contact with angle B.
In this attached image, you can see better that sides AB and BC is in contact with angle B. So, the opposite side to angle B is AC.
Therefore, the lenght of the side opposite to angle B is 4 units.