Interpret the energy profile

In a reaction coordinate diagram:

• The vertical axis represents potential energy.
• The horizontal axis represents reaction progress.
• The peak corresponds to the activated complex (transition state).

Let:

• Reactants be at some initial energy level.
• Products be at a higher or lower final energy level.
• The peak be above both.

Meaning of $E_1$ and $E_2$

From the standard labeling of such diagrams:

• $E_1$ is the energy difference between products and reactants.
• $E_2$ is the energy difference between the transition state and products.

Therefore, the energy difference between transition state and reactants is: $E_a(\text{forward}) = E_1 + E_2$

Find stability of reactants and products

Stability is inversely related to energy:

• Lower energy $\Rightarrow$ more stable
• Higher energy $\Rightarrow$ less stable

Since the products lie above the reactants by $E_1$, the products have higher energy. Hence, products are less stable than reactants.

Check each option

A: Activation energy of both forward and backward reaction is $E_1+E_2$ and reactant is more stable than product.

• Forward activation energy can be $E_1+E_2$, but backward activation energy is not the same.
• So this is false.

B: Activation energy of forward reaction is $E_1+E_2$ and product is more stable than reactant.

• Second statement is false. Products are less stable.
• So this is false.

C: Activation energy of backward reaction is $E_1$ and product is more stable than reactant.

• Backward activation energy should be the energy gap from products to transition state, i.e. $E_2$, not $E_1$.
• Also products are not more stable.
• So this is false.

D: Activation energy of forward reaction is $E_1+E_2$ and product is less stable than reactant.

• Both statements are correct.

Final answer

The correct option is: $\boxed{D}$