B

Step 1: Calculate the relative speed of the two trains. Since they are moving in opposite directions, their speeds add up.
If train 1 is moving at 10 m/s and train 2 at 15 m/s, the relative speed \( V_{relative} = V_1 + V_2 = 10 + 15 = 25 \text{ m/s}
Step 2: Calculate the total length of the two trains.
Length of train 1 = 50 m, Length of train 2 = 50 m.
Total distance to be covered = 50 + 50 = 100 m.
Step 3: Use the formula for time, \( t = \frac{distance}{speed} \).
Therefore, \( t = \frac{100 \text{ m}}{25 \text{ m/s}} = 4 \text{ s}.
Hence, the trains take 4 seconds to cross each other.