E1cb (Elimination Unimolecular Conjugate Base) is a type of elimination reaction that occurs in a two-step mechanism. Let's go through each statement and explain why it is true.

a) The overall order of reaction is 2:
In the E1cb reaction, the overall order of the reaction is determined by the rate-determining step, which is the first step of the mechanism. This step involves the formation of a carbanion intermediate by the removal of a leaving group. Since this step is unimolecular, its rate depends only on the concentration of the substrate (or the leaving group). Therefore, the overall reaction order is 2, as it only depends on the concentration of the substrate.

b) Electron withdrawing group adjacent to the carbon favor this mechanism:
In the E1cb reaction, the presence of an electron-withdrawing group (EWG) adjacent to the carbon that bears the leaving group helps stabilize the carbanion intermediate. The EWG can withdraw electron density from the carbon-carbon double bond, which stabilizes the negative charge on the carbanion. This stabilization lowers the energy barrier for the formation of the carbanion intermediate and favors the E1cb mechanism.

c) The rate-determining step is unimolecular:
The rate-determining step in the E1cb reaction is the formation of the carbanion intermediate. This step involves the removal of a leaving group and the generation of a carbanion. Since this step only involves one molecule (the substrate), it is considered unimolecular. The rate of the reaction is determined solely by the concentration of the substrate and not by any other molecules present in the reaction mixture.

d) If deuterated solvent is used, deuterium incorporation in the substrate may be observed:
When a deuterated solvent is used in the E1cb reaction, deuterium atoms (D) can be incorporated into the substrate. This occurs because the carbanion intermediate can abstract a proton from the deuterated solvent, leading to the exchange of hydrogen (H) with deuterium (D). Therefore, if the reaction is carried out in a deuterated solvent, the resulting product may contain deuterium atoms, which can be observed through NMR spectroscopy.

In summary, all of the given statements about the E1cb reaction are true. The overall reaction order is 2, electron-withdrawing groups adjacent to the carbon favor this mechanism, the rate-determining step is unimolecular, and deuterium incorporation can be observed if a deuterated solvent is used.