To solve the equation \( a^2 - 2a + 1 = 0 \), we can start by factoring the quadratic expression.
Notice that \( a^2 - 2a + 1 \) can be factored as:
\[ (a - 1)^2 = 0 \]
Setting the factor equal to zero gives us:
\[ a - 1 = 0 \]
This simplifies to:
\[ a = 1 \]
Thus, the solution set is:
\[ { 1 } \]
So the correct choice is:
A. The solution set is \({ 1 }\).