Reactions which involve absorption of heat energy are called endothermic reactions. For such reactions
$\Sigma H_{P} >\Sigma H_{R}$
In graph (c) and (d) the heat of products is more than heat of reactants
$\therefore $ They represents endothermic reaction. But in (d) only small amount of energy is absorbed (less difference between energy of reactants and products). Thus, (c) represent maximum activation energy.
Activation energy is the excess energy that the reactant molecule must possess to cross energy barrier.