For a convex polygon with $n$ edges $F$, if all its diagonals have the equal length, then
$\text{(A)}F\in \{\text{quadrilaterals}\}$
$\text{(B)}F\in \{\text{pentagons}\}$
$\text{(C)}F\in \{\text{pentagons}\} \cup\{\text{quadrilaterals}\}$
$\text{(D)}F\in \{\text{convex polygons that have all edges' length equal}\} \cup\{\text{convex polygons that have all inner angles equal}\}$
1. **Understanding the Problem:**
   We are given a convex polygon with \( n \) edges, and we need to determine the nature of the polygon if all its diagonals have equal length.

2. **Analyzing the Diagonals:**
   - For a polygon to have all diagonals of equal length, it must have a high degree of symmetry.
   - Regular polygons (where all sides and all angles are equal) are the most symmetric polygons.

3. **Considering Regular Polygons:**
   - In a regular polygon, all sides and angles are equal, and the diagonals are also of equal length.
   - For \( n = 4 \) (a square), all diagonals are equal.
   - For \( n = 5 \) (a regular pentagon), all diagonals are equal.
   - For \( n = 6 \) (a regular hexagon), not all diagonals are equal. The diagonals of different lengths are present.

4. **Conclusion:**
   - The only regular polygons where all diagonals are equal are the square (quadrilateral) and the regular pentagon.
   - Therefore, the polygon can either be a quadrilateral or a pentagon.

The final answer is \( \boxed{ \text{(C)}F\in \{\text{pentagons}\} \cup\{\text{quadrilaterals}\} } \)