Consider six squares with sides of different lengths. The length of the side of a square is proportional to:
Perimeter of the square
Both of these
If two things are proportional that indicates their ratio will be constant. Now, let's verify options.
Length of squarePerimeter of square=a4a=14
which is a constant.
Length of squareArea of square=aa2=1a
which is not constant and varies with side of square.
Length of squareLength of diagonal=a√2a=1√2
which is constant.
Hence, the length of the side of a square is proportional to its perimeter and diagonal.