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650 meter

555 meter

500 meter

458 meter

C. 500 meter

Let length of tunnel is x meter

Distance = 800+x meter

Time = 1 minute = 60 seconds

Speed = 78 km/hr = 78*5/18 m/s = 65/3 m/s

Distance = Speed*Time

$\Rightarrow 800+x=\frac{65}{3}\times 60\phantom{\rule{0ex}{0ex}}\Rightarrow 800+x=20\times 65=1300\phantom{\rule{0ex}{0ex}}\Rightarrow x=1300-800=500$

So the length of the tunnel is 500 meters.