1. **State the problem:** We need to find the size of an interior angle $c$ of a regular pentagon. 2. **Recall the formula for the interior angle of a regular polygon:** The measure of each interior angle in a regular polygon with $n$ sides is $$\text{Interior angle} = \frac{(n-2) \times 180^\circ}{n}.$$ 3. For a pentagon, $n=5$. Substitute $n=5$ into the formula: $$c = \frac{(5-2) \times 180^\circ}{5} = \frac{3 \times 180^\circ}{5} = \frac{540^\circ}{5} = 108^\circ.$$ 4. **Conclusion:** The interior angle $c$ of the regular pentagon is $108^\circ$.