The maximum number of explosive unit wagons in a goods train is 3. This is because explosive unit wagons are classified as dangerous goods, and there are strict regulations on the transportation of dangerous goods. These regulations limit the number of dangerous goods wagons that can be carried in a train, and the type of wagons that can be carried together.
Option A is incorrect because it is more than the maximum number of explosive unit wagons that can be carried in a train. Option B is incorrect because it is more than the maximum number of explosive unit wagons that can be carried in a train. Option C is correct because it is the maximum number of explosive unit wagons that can be carried in a train. Option D is incorrect because it is not one of the options.