The leaving group is that functional group which is ejected with `overline e^, s` of the `sigma` -bond in a reaction. Better the leaving group, faster is the reaction. The relative leaving ability of the leaving group `X` in `(R - X)` is increased by :
(i) The polarisability of `(R - X)` bond.
(ii) The stability of `X^(Ө)`.
(iii) The degree of stabilisation through solvation of `X`.
(iv) The strength of `(R - X)` bond.
The leaving group tendency is also called fugacity.
Which statement is correct ?

Options

A. `EtO^(Ө)` is a strong base and therefore a good leaving group.
B. The amine group in `ArNH_2` is converted into a good leaving group be reacting `ArNH_2` with `NaNO_2 + HCl` at `0^@ C`.
C. The `(OH)` group is converted into a good leaving group by reacting alcohols with `TsCl` (p-toluene sulphonyl chloride)
D. The amine group in `RNH_2` is converted into leaving group by reacting `RNH_2` with `NaNO_2 + HCl` at `0^@ C`.

Explanation

To determine which statements regarding leaving groups are correct, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement A**: "Ethoxide ion is a strong base and therefore a strong leaving group."
- **Analysis**: A good leaving group must be a weak base. Ethoxide ion (C2H5O-) is a strong base, which means it is not a good leaving group because it would tend to re-attack the substrate rather than leave. Therefore, this statement is **incorrect**.

2. **Statement B**: "The amine group in ArNH2 is converted into a good leaving group on reacting ArNH2 with NaNO2 at Cl at 0°C."
- **Analysis**: When aniline (ArNH2) reacts with sodium nitrite (NaNO2) in the presence of hydrochloric acid at low temperatures, it forms diazonium chloride (ArN2+Cl-). The leaving group here is the nitrogen gas (N2), which is a very stable and weak base. Thus, this statement is **correct**.

3. **Statement C**: "The OH group is converted into a good leaving group by reacting alcohol with tosyl chloride."
- **Analysis**: Alcohols (R-OH) are generally poor leaving groups. However, when they react with tosyl chloride (TsCl), they form tosylate (R-OTs), which is a much better leaving group. This is because the tosylate ion is more stable than the hydroxide ion. Therefore, this statement is **correct**.

4. **Statement D**: "The amine group in RNH2 is converted into a good leaving group by reacting amine with NaNO2 HCl at 0°C."
- **Analysis**: Similar to statement B, the reaction of RNH2 with NaNO2 and HCl leads to the formation of a diazonium ion (RN2+). The leaving group here is again nitrogen gas (N2), which is stable and a good leaving group. Thus, this statement is **correct**.

### Conclusion:
The correct statements are B, C, and D. Therefore, the answer is **BCD**.