Decreasing order of acid strengths is :
`underset((A))(Ph-OH)," "underset((B))(Ph-CH_2-OH)," "underset((C))(Ph-CO_2H)," "underset((D))(Ph-CH_2-overset(o+)(NH_3)),`

Options

A. `B gt A gt C gt D`
B. `C gt A gt B gt D`
C. `C gt A gt D gt B`
D. `C gt B gt A gt D`

Explanation

To determine the decreasing order of acid strengths among the given compounds, we need to analyze the stability of their conjugate bases. The stronger the acid, the more stable its conjugate base will be. Here's the step-by-step solution:

### Step 1: Identify the Compounds and Their Conjugate Bases
1. **(A) Ph-OH** (Phenol) → Conjugate base: Ph-O⁻ (Phenoxide ion)
2. **(B) Ph-CH₂-OH** (Benzyl alcohol) → Conjugate base: Ph-CH₂-O⁻
3. **(C) Ph-CO₂H** (Benzoic acid) → Conjugate base: Ph-COO⁻ (Benzoate ion)
4. **(D) Ph-CH₂-NH₃⁺** (Benzylammonium ion) → Conjugate base: Ph-CH₂-NH₂ (Benzylamine)

### Step 2: Analyze the Stability of the Conjugate Bases
- **(A) Ph-O⁻**: The phenoxide ion is stabilized by resonance. The negative charge can be delocalized over the aromatic ring, making it relatively stable.

- **(B) Ph-CH₂-O⁻**: The conjugate base here lacks resonance stabilization since the negative charge on the oxygen cannot be delocalized into the aromatic ring. This makes it less stable compared to the phenoxide ion.

- **(C) Ph-COO⁻**: The benzoate ion is highly stabilized by resonance. The negative charge can be delocalized over two oxygen atoms, leading to multiple resonance structures. This makes it the most stable conjugate base among the options.

- **(D) Ph-CH₂-NH₂**: This is a neutral amine and does not have a negative charge. Neutral compounds are generally less acidic than their charged counterparts since they do not readily donate protons.

### Step 3: Determine the Order of Acid Strength
Based on the stability of the conjugate bases:
1. **(C) Ph-CO₂H** (Benzoic acid) - Most acidic due to the resonance stabilization of the benzoate ion.
2. **(A) Ph-OH** (Phenol) - Second most acidic due to the resonance stabilization of the phenoxide ion.
3. **(B) Ph-CH₂-OH** (Benzyl alcohol) - Less acidic due to lack of resonance stabilization in its conjugate base.
4. **(D) Ph-CH₂-NH₃⁺** (Benzylammonium ion) - Least acidic since it is neutral and does not readily donate a proton.

### Final Order
Thus, the decreasing order of acid strengths is:
**C > A > B > D**