The correct option is A 12 km/h, 4 km/h
Let the direction of flow of river as +ve.
swimmer's speed in still water means speed of swimmer w.r.t river flow,
vs,r=vs−vr ...(i)
given that
where vs= speed of swimmer w.r.t ground
vr= speed of river flow w.r.t ground
⇒When swimming in direction of river flow
vs=+16 km/h
∴vs,r=16−vr ...(ii)
⇒When swimming in direction opposite of river flow
(∵ direction of swimmer will be −ve)
vs=−8 km/h
∴−vs,r=−8−(+vr) ...(iii)
Adding Eq (ii) & (iii),
−2vr=−8
∴vr=4 km/h
Hence, from Eq (i),
vs,r=16−4=12 km/h
∴ option (a) is correct.