What is the collector-emitter resistance of NPN Transistor?
The BJT collector current equation is

$$i_C = I_S\cdot e^{\frac{v_{BE}}{V_T}}\left(1 + \frac{v_{CB}}{V_A}\right)$$

where \$V_A\$ is the Early voltage.  But, this formula is often written as

$$i_C = I_S\cdot e^{\frac{v_{BE}}{V_T}}\left(1 + \frac{v_{CE}}{V_A}\right)$$

Thus

$$\frac{\partial i_C}{\partial v_{CE}} =  \frac{I_S\cdot e^{\frac{V_{BE}}{V_T}}}{V_A} = \frac{i_C}{V_A + v_{CE}}$$

This is clearly a non-linear function of the collector-emitter voltage and collector current so this cannot be interpreted as a conductance.

However, for small changes around some fixed value of collector current \$I_C\$ and collector-emitter voltage \$V_{CE}\$, we can write

$$I_C + i_c \approx I_C\left(1 + \frac{v_{ce}}{V_A + V_{CE}} \right) = I_C + \frac{v_{ce}}{r_o}$$

where

$$r_o = \frac{V_A + V_{CE}}{I_C}$$

We call \$r_o\$ the collector-emitter dynamic, or differential or small-signal resistance.

It is not a true resistance since it is not constant but, instead, varies with the operating point of the transistor as can be seen by the formula.