#### Answer

(a) Since the dot product gives scalar then the expression
$$(\mathbf{u} \cdot \mathbf{v}) \cdot \mathbf{u}$$
is meaningless, that is, one can not consider the dot product of a scalar and a vector.
(b Since the dot product gives scalar then the expression
$$c \cdot(\mathbf{u} \cdot \mathbf{v})$$
is meaningless, that is, one can not consider the dot product of two scalars.