The advantages of a 25-kV, 50 Hz AC system are:

1. Less number of substations: A 25-kV, 50 Hz AC system allows for longer distances between substations, reducing the need for a large number of substations. This can result in lower infrastructure costs and greater efficiency in the transmission of electricity.

2. High flexibility: AC systems are generally more flexible than DC systems, allowing for easier adjustments in voltage and frequency to accommodate changes in demand and supply. This makes a 25-kV, 50 Hz AC system more adaptable to changing conditions and more reliable in operation.

3. High coefficient of adhesion: AC traction systems, such as those used in railways, have a high coefficient of adhesion, which means that they can provide high tractive effort and better traction on the tracks. This can result in faster and more efficient movement of trains, reducing journey times and increasing capacity.

Therefore, the correct answer to the question is option D, "All of the above".