6) A

‘a’ is the energy change.

‘b’ is the energy of reactants

‘c’ is the activation energy

‘e’ is energy of products

Here, the energy of products is more than energy of reactants.
Therefore, (products – reactants) would give a positive value. Here it is labelled ‘a’

In the reverse reaction, energy of reactants would be more than energy of products.

Therefore, (products – reactants) would give a negative but equal value. Therefore, it would be ‘-a’