44 km/hour

46 km/hour

48 km/hour

50 km/hour

C. 48 km/hour

Speed while going = 40 km/hr

Speed while returning = 150% of 40 = 60 km/hr

Average speed =$\frac{2xy}{x+y}$

$=\frac{2\times 40\times 60}{40+60}=\frac{4800}{100}\phantom{\rule{0ex}{0ex}}=48Km/hr$