Correct Answer: 5 km.

A is the starting point and

B is the finishing point.

To find the distance between A and B

From \( \Large \triangle ADC, \angle B = 90 ^{\circ} ;\ AC = 3 km.\ BC = 4 km. \)

\( \Large AB^{2} = AC^{2} + BC^{2} \)

=\( \Large 3^{2} + 4^{2} \)

=\( \Large 9 + 16 =25 \)

\( \Large AB = \sqrt{25} = 5 \)