Magnetomotive force (mmf or simply F) is a measure of the strength of magnetic field in a coil. Conceptually it is analogous to electromotive force in a resistor circuit. For a given length of a coil, mmf can be increased by increasing the number of turns in the circuit.
The magnitude of magnetomotive force is obtained by multiplying the number of turns with the current strength.
F = N*I
Here, N = 250, I = 0.4 amperes
So, F= 250*0.4 = 100
Therefore, the mmf of the coil is 100 ampere-turns.