Answer
(a) True
(b) False
(c) False
Work Step by Step
(a) This is true and we can always solve it by equating one of the variables with arbitrary parameter $t$ and express the other variable in terms of $t$ and those expressions will list all of the possible solutions.
(b) This is not true. The two equations may not be consistent with each other. Let us show this by example. Consider the system
\begin{align*}
x+y+z=&5\\
x+y+z=&6
\end{align*}
This system is inconsistent because since left sides are equal, right sides have to be also but obviously $5\neq 6$.
(c) This is false. The system can also have unique solution.
