Correct Answer: 6

Solution : Let downstream speed and upstream speeds be $u$ km/hr and $v$ km/hr, respectively.
Dharmendra can row 80 km upstream and 110 km downstream in 13 hours
$\frac{110}{u}+\frac{80}{v}=13$..................(I)
He can row 60 km upstream and 88 km downstream in 10 hours.
$\frac{88}{u}+\frac{60}{v}=10$..................(II)
Equation (I) is multiplied by 3 and equation (II) is multiplied by 4.
We get,
$\frac{330}{u}+\frac{240}{v}=39$..................(III)
$\frac{352}{u}+\frac{240}{v}=40$..................(IV)
Subtracting equation (III) from equation (IV), we get,
$\frac{22}{u}=1$
⇒ $u=22$ km/hr
Putting $u = 22$ in equation (I), we get,
$\frac{110}{22}+\frac{80}{v}=13$
⇒$\frac{80}{v}=8$
⇒$v=10$ km/hr
Speed of the current = $\frac{1}{2}$ × (Downstream speed – Upstream speed)
= $\frac{1}{2}$ (22 – 10) = 6 km/hr
Hence, the correct answer is 6.