A person crosses a diagonal of a square field in 3 seconds at the speed of 6 km./hr. The area of the field is square mts.

Correct option is D

Given:
Time taken to cross the diagonal = 3 seconds
Speed = 6 km/h
Formula Used:

Distance = Speed × Time

Diagonal = Side $$\times \sqrt{2}$$​​

Area = $$\text{Side}^2$$

Solution:

Convert speed to m/s: $$6 \times \frac{5}{18} = \frac{5}{3}$$ m/s

Distance = $$\frac{5}{3}\times 3 = 5$$ m

5 = Side $$\times \sqrt{2}$$

Side = $$\frac{5}{\sqrt{2}}$$

Area = $$\left(\frac{5}{\sqrt{2}}\right)^2 = \frac{25}{2} = 12.5$$ square meters

Thus, the correct answer is (d).