Final answer:
In a parallelogram, opposite sides are equal. Since QS = 10, the length of RT must also be 10, making option a) the correct answer.
Step-by-step explanation:
The question involves finding the length of the side RT in a parallelogram QRST. In geometry, it is known that opposite sides of a parallelogram are equal in length. Since QS and RT are opposite sides, the length of RT must be equal to the length of QS. Given that QS = 10 units, it follows that RT must also be 10 units. Therefore, the correct answer to the question is a) 10.