b
(b) Let $\frac{{{x^2}}}{{{a^2}}} = X,\frac{{{y^2}}}{{{b^2}}} = Y$and $\frac{{{z^2}}}{{{c^2}}} = Z$, .
then the given system of equations is $X + Y - Z = 1,$ $X - Y + Z = 1$, $ - X + Y + Z = 1$.
The coefficient matrix is $A = \left[ {\begin{array}{*{20}{c}}1&1&{ - 1}\\1&{ - 1}&1\\{ - 1}&1&1\end{array}} \right]$
Clearly $|A| \ne 0$. So the given system of equations has unique solution.