The electric force between charged particles is inversely proportional to the square of the distance between them, according to Coulomb's Law:
\[ F \propto \frac{1}{r^2} \]
where \( F \) is the electric force and \( r \) is the distance between the charges. This means that the smaller the distance, the greater the electric force.
Looking at the table:
- Pair A: 16 units
- Pair B: 24 units
- Pair C: 11 units
- Pair D: 29 units
Among these distances, Pair C has the smallest distance (11 units).
Thus, Pair C has the strongest electric force.
The answer is D. Pair C.