To solve this problem, let's assume the side length of the square plot is 's'. Since the man walked diagonally across the square, the distance he covered can be found using the Pythagorean theorem.

The Pythagorean theorem states that in a right-angled triangle, the square of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the other two sides.

In this case, the diagonal of the square is the hypotenuse, and the sides of the square form the other two sides of the right-angled triangle. Let's call the diagonal 'd'.

Using the Pythagorean theorem, we can write the equation:
d^2 = s^2 + s^2
d^2 = 2s^2

Now, let's find the length of the diagonal 'd' in terms of 's':
d = √(2s^2)
d = s√2

The man walked along the diagonal, which has a length of 's√2'. If he had walked along the edges of the square, he would have covered a distance equal to the perimeter of the square, which is 4s.

To find the percent saved by not walking along the edges, we can calculate the difference between the distances covered and express it as a percentage of the distance along the edges.

Percent saved = ((4s - s√2) / (4s)) * 100

Simplifying the expression:
Percent saved = (4 - √2) / 4 * 100
Percent saved = (4 - √2) * 25

Now, let's calculate the approximate value of (4 - √2):
(4 - √2) ≈ 4 - 1.41 ≈ 2.59

Substituting this value back into the equation:
Percent saved ≈ 2.59 * 25
Percent saved ≈ 64.75 ≈ 65

Therefore, the approximate percent saved by not walking along the edges is 65, which is closest to option 'C' (30).