Answer: The correct option is (C) 2 units.
Step-by-step explanation: Given that the sides BD and DC have the same length in the quadrilateral ABCD.
We are to find the length of side BC.
From the figure, we note that
triangle ABD is a right-angled triangle with BD as the hypotenuse. So, using Pythagoras theorem, we get

Now, according to the given information, we have

Again, triangle BCD is a right-angled triangle with BC as the hypotenuse. So, using Pythagoras theorem, we get

Thus, the length of side BC is 2 units.
Option (C) is CORRECT.