To determine the decreasing order of basic strength of the given conjugate bases, we need to understand the concept of basicity. A base is defined by its ability to accept protons (H⁺). The stronger the conjugate base, the more readily it accepts protons. The basicity of conjugate bases is also influenced by the stability of the anion (conjugate base); the more stable the anion, the weaker the base, and vice versa.

Let's analyze each conjugate base:

1. $${ }^{-} \mathrm{OH}$$ (Hydroxide ion): This is a strong base because it is less stabilized by resonance or inductive effects.

2. $$\mathrm{R} \overline{\mathrm{O}}$$ (Alkoxide ion): This also is a strong base. However, alkyl groups (R) are electron donating, which can increase the electron density on oxygen, making it a stronger base compared to hydroxide ion.

3. $$\mathrm{CH}_3 \mathrm{CO} \overline{\mathrm{O}}$$ (Acetate ion): This is a much weaker base because the negative charge is delocalized over the two oxygen atoms through resonance, which stabilizes the ion.

4. $$\mathrm{Cl}$$ (Chloride ion): Chloride ion is very weakly basic since it is highly stable due to its high electronegativity and full octet.

Based on these observations, the decreasing order of basic strength should be:

$$\mathrm{R} \overline{\mathrm{O}}>^{-} \mathrm{OH}>\mathrm{CH}_3 \mathrm{CO} \overline{\mathrm{O}}>\mathrm{Cl}$$

Therefore, the correct choice is:

Option D

$$\mathrm{R} \overline{\mathrm{O}}>{ }^{-} \mathrm{OH}>\mathrm{CH}_3 \mathrm{CO} \overline{\mathrm{O}}>\mathrm{Cl}$$