L, M, N, O and P are five buses of different sizes. Two of them provide food on board, 3 are luxurious buses and 3 are air conditioned buses. One has capacity of less than 20 passengers, 2 have capacity between 20 to 40 passengers, 2 have capacity of more than 40 passengers. The buses having capacity of more than 40 passengers are air-conditioned but not luxurious. Bus N has capacity of less than 20 passengers and is luxurious. Buses L and O provide food. The buses which provide food are luxurious and ply between Delhi and Agra.

The buses which ply between Delhi and Agra are:

​Buses: L, M, N, O, P (five different buses).
Bus types:
2 provide food on board.
3 are luxurious.
3 are air-conditioned

Capacity:
1 bus has capacity less than 20 passengers.
2 buses have a capacity between 20 and 40 passengers.
2 buses have capacity more than 40 passengers.
Conditions:
The buses with more than 40 passengers are air-conditioned but not luxurious.
Bus N has a capacity of less than 20 passengers and is luxurious.
Buses L and O provide food.
The buses providing food are luxurious and ply between Delhi and Agra.

From the given information, it is clear that thr buses L and O provide food on board, and the buses that provide food ply between Delhi and Agra.
Therefore, the buses that ply between Delhi and Agra are L and O.