Decreasing order of acid strength is : `Phunderset((A))-OH," " Ph-underset((B))CH_(2)-OH," "Ph-CO_(2)H," " Ph-underset((D))CH_(2)-overset(o+)NH_(3)`

Options

A. `BgtAgtCgtD`
B. `CgtAgtBgtD`
C. `CgtAgtDgtB`
D. `CgtBgtAgtD`

Explanation

To determine the decreasing order of acid strength for the compounds given, we need to analyze the stability of the conjugate bases formed when each compound donates a proton (H+). The more stable the conjugate base, the stronger the acid.

### Step-by-Step Solution:

1. **Identify the Compounds:**
- (A) Ph-OH (Phenol)
- (B) Ph-CH2-OH (Benzyl alcohol)
- (C) Ph-CO2H (Benzoic acid)
- (D) Ph-CH2-NH3+ (Benzylammonium ion)

2. **Determine the Conjugate Bases:**
- For (A) Ph-OH, the conjugate base is Ph-O⁻ (phenoxide ion).
- For (B) Ph-CH2-OH, the conjugate base is Ph-CH2-O⁻ (benzyl oxide ion).
- For (C) Ph-CO2H, the conjugate base is Ph-CO2⁻ (benzoate ion).
- For (D) Ph-CH2-NH3+, the conjugate base is Ph-CH2-NH2 (benzylamine).

3. **Analyze the Stability of Conjugate Bases:**
- **(A) Ph-O⁻:** The negative charge on oxygen can be delocalized due to resonance with the aromatic ring, making it relatively stable.
- **(B) Ph-CH2-O⁻:** The negative charge is localized on the oxygen and does not have resonance stabilization from the aromatic ring, making it less stable than Ph-O⁻.
- **(C) Ph-CO2⁻:** The negative charge is delocalized over two oxygen atoms in the carboxylate ion, making it very stable and thus, benzoic acid is a strong acid.
- **(D) Ph-CH2-NH2:** This is a neutral amine and does not have a negative charge, making it the least acidic.

4. **Rank the Acids:**
- **Strongest Acid:** (C) Ph-CO2H (Benzoic acid) - due to the resonance stabilization of the conjugate base.
- **Next Strongest:** (A) Ph-OH (Phenol) - resonance stabilization of phenoxide ion.
- **Next:** (B) Ph-CH2-OH (Benzyl alcohol) - no resonance stabilization.
- **Weakest Acid:** (D) Ph-CH2-NH3+ (Benzylammonium ion) - neutral and not acidic.

5. **Final Order:**
- The decreasing order of acid strength is: **Ph-CO2H > Ph-OH > Ph-CH2-OH > Ph-CH2-NH3+**.

### Final Answer:
The order of decreasing acid strength is: **C > A > B > D**.