Activation energy is the `underline("sum ")`of threshold energy and energy of reactants.

Explanation

To solve the question regarding the relationship between activation energy, threshold energy, and the energy of reactants, we can break it down step by step.

### Step-by-Step Solution:

1. **Understand the Definitions**:
- **Activation Energy (Ea)**: This is the minimum energy required for a reaction to occur. It is the extra energy needed by the reactants to reach the transition state and form products.
- **Threshold Energy (Et)**: This is the minimum energy that reactants must possess for a reaction to take place.
- **Energy of Reactants (Er)**: This is the inherent energy that the reactants possess before the reaction begins.
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