Give an example of:
$(a)$ a physical quantity which has a unit but no dimensions
$(b)$ a physical quantity which has neither unit nor dimensions
$(c)$ a constant which has a unit
$(d)$ a constant which has no unit

$(a)$ Plane angle $\theta = \frac{l}{r}$ has the unit radian, but its dimensional formula is $[M^{0} L^{0} T^{0}]$.
$(b)$ Strain $= \frac{\Delta l}{l} = \frac{\text{Change in length}}{\text{Original length}}$. It has neither a unit nor dimensions.
$(c)$ Gravitational constant $G = 6.67 \times 10^{-11} \text{ N} \cdot \text{m}^{2} \cdot \text{kg}^{-2}$. It is a constant with a unit.
$(d)$ Reynolds number $(Re)$ is a dimensionless constant.