To find the average speed of the train in meters per second, we need to follow these steps:

First, let's convert the distance from kilometers to meters. We know that 1 kilometer is equal to 1000 meters. Given that the distance is 756 km, we convert it to meters:

1. \(756 \text{ km} = 756 \times 1000 \text{ m} = 756000 \text{ m}\)

The time taken by the train is given as 14 hours. We need to convert this time into seconds, as we need the speed in meters per second. There are 3600 seconds in an hour. So, the conversion is:

1. \(14 \text{ hours} = 14 \times 3600 \text{ seconds} = 50400 \text{ seconds}\)

Now, we can calculate the average speed using the formula:

1. \(\text{Average Speed} = \frac{\text{Total Distance}}{\text{Total Time}}\)

Substitute the values we have into the formula:

1. \(\text{Average Speed} = \frac{756000 \text{ m}}{50400 \text{ sec}}\)

Performing the division, we get:

1. \(\text{Average Speed} = 15 \text{ m/s}\)

Hence, the average speed of the train is 15 m/s. Therefore, the correct answer is 15 m/s.