Let $A'$ be the result of reflection of vertex $A$ of triangle ABC through line $BC$ and let $B'$ be the result of reflection of vertex $B$ through line $AC$. Given that $\angle BA' C = \angle BB'C$, can the largest angle of triangle $ABC$ be located:
a) At vertex $A$,
 b) At vertex $B$,
c) At vertex $C$?
1. **Reflecting Vertex \( A \) through Line \( BC \):**
   - When vertex \( A \) is reflected through line \( BC \), the resulting point is \( A' \). The reflection property ensures that \( \angle BA'C = \angle BAC \).

2. **Reflecting Vertex \( B \) through Line \( AC \):**
   - Similarly, when vertex \( B \) is reflected through line \( AC \), the resulting point is \( B' \). The reflection property ensures that \( \angle BB'C = \angle ABC \).

3. **Given Condition:**
   - We are given that \( \angle BA'C = \angle BB'C \).

4. **Using the Reflection Properties:**
   - From the reflection properties, we know:
     \[
     \angle BA'C = \angle BAC
     \]
     \[
     \angle BB'C = \angle ABC
     \]

5. **Equating the Angles:**
   - Given \( \angle BA'C = \angle BB'C \), we can write:
     \[
     \angle BAC = \angle ABC
     \]

6. **Analyzing the Triangle:**
   - In triangle \( ABC \), the sum of the angles is \( 180^\circ \):
     \[
     \angle BAC + \angle ABC + \angle ACB = 180^\circ
     \]
   - Since \( \angle BAC = \angle ABC \), let \( \angle BAC = \angle ABC = x \). Then:
     \[
     x + x + \angle ACB = 180^\circ
     \]
     \[
     2x + \angle ACB = 180^\circ
     \]
     \[
     \angle ACB = 180^\circ - 2x
     \]

7. **Determining the Largest Angle:**
   - To determine the largest angle, we compare \( x \) and \( 180^\circ - 2x \):
     - If \( x \) is the largest angle, then \( x > 180^\circ - 2x \):
       \[
       3x > 180^\circ
       \]
       \[
       x > 60^\circ
       \]
     - If \( 180^\circ - 2x \) is the largest angle, then \( 180^\circ - 2x > x \):
       \[
       180^\circ > 3x
       \]
       \[
       x < 60^\circ
       \]

8. **Conclusion:**
   - Since \( x \) and \( 180^\circ - 2x \) are angles in a triangle, \( x \) must be less than \( 90^\circ \). Therefore, the largest angle must be \( 180^\circ - 2x \).

The final answer is \( \boxed{ \angle ACB } \) (at vertex \( C \)).