1. **Problem Statement:** We need to calculate the size of the interior angle labeled $c$ in a regular pentagon. 2. **Recall properties of a regular pentagon:** A regular pentagon has 5 equal sides and 5 equal interior angles. 3. **Calculate the sum of interior angles:** The formula for the sum of interior angles of a polygon with $n$ sides is: $$\text{Sum of interior angles} = (n-2) \times 180^\circ$$ For a pentagon, $n=5$: $$ (5-2) \times 180^\circ = 3 \times 180^\circ = 540^\circ $$ 4. **Calculate each interior angle:** Since all 5 interior angles are equal, $$ c = \frac{540^\circ}{5} = 108^\circ $$ 5. **Answer:** The size of the interior angle marked $c$ in the regular pentagon is $108^\circ$.