Explanation:

- Pinky can swim downstream 27 km in 3 hours, which implies her downstream speed is 27 km / 3 hrs = 9 km/h.

- Pinky can swim upstream 15 km in 3 hours, meaning her upstream speed is 15 km / 3 hrs = 5 km/h.

- Let "b" represent her swimming speed in still water and "c" the current's speed.

- Downstream, the effective speed = $b+c$; Upstream, the effective speed = $b-c$.

- We have these equations:

- $b+c=9$

- $b-c=5$

- Adding those equations:

- 2b = 14

- b = 7

- Subtracting the upstream equation from the downstream equation gives:

- 2c = 4

- c = 2 km/h

- Thus, the current's velocity is 2 km/h.

Answer: Option 4 - 2 km/h