The answer is D. Although the tension in the couplings will be equal for opposite ends of the same coupling, it will not be the same between all the couplings on the train. Each train car most likely has a different mass, but all are traveling at the same acceleration. Cars that have more mass will require more force to keep them moving at the same acceleration as another car in the train with less mass. So the tension in the couplings will vary depending on the individual masses of the train cars, and the amount of force that will be required to keep them at the trains acceleration.

we don't if the masses of the cars are equal or not.

It's D because in order to use the equation f=ma I need also need to know the mass of each car besides knowing theat each car has the same acceleration.