The power grid is the system that transfers electricity to homes and businesses. a power plant produces electricity. the output electricity is an input for the wires that lead to power sub stations. the power substations regulate the flow of electricity and send it as an output through different wires. transformers adjust the electricity along the way until it becomes an input for a home. what is the primary type of input and output that is used in the power grid system

First of all, input electricity is just indicating the direction of that energy, in this case, it refers to the  electrity that is being received in a sub-system. On the other hand, output electricity is the energy that is being produced to another sub-system.

For example, in this case the power plant represents the production of energy, which offer output electricity because produces the energy for other sub-system. Then, those sub-systems receive this energy, that's why at this level it becomes input energy.

Being said that, we can say that energy is the primary type of input and output, it's what is being used in the power grid.

think its the third option

option number c is the right answer.