$17\frac{6}{7}$ km/hr

25 km/hr

30 km/hr

35 km/hr

D. 35 km/hr

Time taken = 1 hr 40 min 48 sec = $1hr40\frac{4}{5}min=1\frac{51}{75}hrs=\frac{126}{75}hrs.$

Let the actual speed be x km/hr.

Then, $\frac{5}{7}x\times \frac{126}{75}=42$

$\Rightarrow x=\left(\frac{42\times 7\times 75}{5\times 126}\right)=35km/hr$