The correct order of decreasing acid strength of trichloroacetic acid (A), trifluoroacetic acid
(B) acetic acid
(C) and formic acid
(D) is:

Options

A. I gt II gt III gt IV
B. I gt III gt II gt IV
C. II gt I gt IV gt III
D. II gt IV gt III gt I

Explanation

To determine the correct order of decreasing acid strength of trichloroacetic acid (A), trifluoroacetic acid (B), acetic acid (C), and formic acid (D), we need to analyze the structure and the effects of the substituents on the acidity of these compounds.

### Step-by-Step Solution:

1. **Identify the Structures**:
- **Trichloroacetic Acid (A)**: CCl₃COOH
- **Trifluoroacetic Acid (B)**: CF₃COOH
- **Acetic Acid (C)**: CH₃COOH
- **Formic Acid (D)**: HCOOH

2. **Understand the Acidity Concept**:
- The strength of an acid is determined by its ability to donate a proton (H⁺). The stability of the conjugate base (the species that remains after the acid donates a proton) is crucial. The more stable the conjugate base, the stronger the acid.

3. **Analyze the Conjugate Bases**:
- When these acids lose a proton, they form:
- **Trichloroacetate (A⁻)**: CCl₃COO⁻
- **Trifluoroacetate (B⁻)**: CF₃COO⁻
- **Acetate (C⁻)**: CH₃COO⁻
- **Formate (D⁻)**: HCOO⁻

4. **Effect of Substituents**:
- **Electron-Withdrawing Groups (EWGs)**: Both chlorine and fluorine are strong electron-withdrawing groups. They stabilize the negative charge on the conjugate base by delocalizing it.
- **Comparison of Strengths**:
- **Trifluoroacetic Acid (B)**: Has three fluorine atoms, which are more electronegative than chlorine, leading to greater stabilization of the conjugate base, making it the strongest acid.
- **Trichloroacetic Acid (A)**: Has three chlorine atoms, which are less electronegative than fluorine, making it the second strongest acid.
- **Formic Acid (D)**: Has no electron-donating groups and is stronger than acetic acid due to the absence of a methyl group which is an electron-donating group.
- **Acetic Acid (C)**: The methyl group (CH₃) is an electron-donating group, which destabilizes the conjugate base, making it the weakest acid among the four.

5. **Conclusion**:
- The order of decreasing acid strength is:
- **Trifluoroacetic Acid (B) > Trichloroacetic Acid (A) > Formic Acid (D) > Acetic Acid (C)**.

### Final Answer:
**B > A > D > C**