a
$f\left( n \right) = {\alpha ^n} + {\beta ^n}$
$\left| {\begin{array}{*{20}{c}}
{1 + 1 + 1}&{1 + \alpha + \beta }&{1 + {\alpha ^2} + {\beta ^2}}\\
{1 + \alpha + \beta }&{1 + {\alpha ^2} + {\beta ^2}}&{1 + {\alpha ^3} + {\beta ^3}}\\
{1 + {\alpha ^2} + {\beta ^2}}&{1 + {\alpha ^3} + {\beta ^3}}&{1 + {\alpha ^4} + {\beta ^4}}
\end{array}} \right|$
$ = \left| {\begin{array}{*{20}{c}}
1&1&1\\
1&\alpha &{{\alpha ^2}}\\
1&\beta &{{\beta ^2}}
\end{array}} \right|$
$ = {\left( {1 - \alpha } \right)^2}{\left( {\alpha - \beta } \right)^2}{\left( {\beta - 1} \right)^2}$
$\Rightarrow \boxed{k = 1}$