Option A: 
Option D: 
Step-by-step explanation:
Option A: 
To find the sides of a square, let us use the distance formula,

Now, we shall find the length of the square,

Thus, the square with vertices
has sides of length a.
Option B: 
Now, we shall find the length of the square,

This is not a square because the lengths are not equal.
Option C: 
Now, we shall find the length of the square,

Thus, the square with vertices
has sides of length 2a.
Option D: 
Now, we shall find the length of the square,

Thus, the square with vertices
has sides of length a.
Thus, the correct answers are option a and option d.