At time t, measured in minutes after the beginning of the time period, which of the following gives the speed of the train in accordance with the table?

Each of the options given assumes the speed of the train to vary linearly wrt time.

So, let us assume the speed of the train at time t be given by 40+ at, because at t=0, the speed was already 40 kmph.

When we put x=30, we get,

45= 40+ 30a

=> 5= 30a Or, a= $$\ \frac{\ 1}{6}\ \ \ $$

So, Speed= 40+ $$\ \frac{\ t}{6}\ \ \ $$

We will check again to verify the results. 

When t=60, Speed= 40+ $$\ \frac{\ 60}{6}\ \ \ $$= 50 kmph, which matches with the value given in the table.

So, Option D is correct