**Concept: The question tests on the ability to solve basic Mensuration problem**

**Solution:**

The perimeter = 140

Taking the variable x, length = 3x and width = 4x

Perimeter = 2(l + b) = 2(3x + 4x) = 14x

14x = 140

x = 10

Length = 30 and width = 40

Length of the diagonal = √l^2+b^2

=√30^2+40^2

=√2500

=50

Option B