Within each pair of elements F and Cl, S and Se, and Li and Na, respectively, the elements that release more energy upon an electron gain are:

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Strategy:
 "Releasing more energy upon electron gain" means having a more negative (exothermic) electron gain enthalpy. Apply  period anomalies and general group trends.

Step 1: Evaluate pairs
- F vs Cl: Chlorine () is more negative than Fluorine () because Fluorine's small size leads to high electron-electron repulsion. (Cl)
- S vs Se: Sulfur () is more negative than Selenium (). (S)
- Li vs Na: Lithium () is slightly more negative than Sodium () due to Lithium's smaller size and stronger nucl

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