Answer
False
Work Step by Step
From Theorem 9.6.1 we have:
$$x_p(t)=X(t)\int^t_0 X^{-1}(S)b(s) ds$$ then integration constants will add to this. Hence there are many particular solutions $x_p$ that satisfy the nonhomogeneous vector differential equation $x′ = Ax + b$.
Hence, the statement is not true.