Arrange the following acids in the decreasing order of their basicities.
(A) Phosphoric acid
(B) Hydrochloric acid
(C) Sulphurous acid

Options

A. ACB
B. ABC
C. BCA
D. CBA

Explanation

To arrange the given acids in decreasing order of their basicities, we need to understand the concept of basicity in acids. Basicity refers to the number of hydrogen ions (H⁺) that an acid can donate when dissolved in water. The more hydrogen ions an acid can donate, the higher its basicity.

Let's analyze each acid:

1. **Phosphoric Acid (H₃PO₄)**:
- This acid has three hydrogen atoms that can ionize.
- Therefore, the basicity of phosphoric acid is **3**.

2. **Hydrochloric Acid (HCl)**:
- This acid has only one hydrogen atom that can ionize.
- Therefore, the basicity of hydrochloric acid is **1**.

3. **Sulphurous Acid (H₂SO₃)**:
- This acid has two hydrogen atoms that can ionize.
- Therefore, the basicity of sulphurous acid is **2**.

Now, we can summarize the basicities:
- Phosphoric Acid: 3
- Sulphurous Acid: 2
- Hydrochloric Acid: 1

Now, we arrange these acids in decreasing order of their basicities:
- Phosphoric Acid (3) > Sulphurous Acid (2) > Hydrochloric Acid (1)

Thus, the final order in decreasing basicity is:
**(A) Phosphoric acid > (C) Sulphurous acid > (B) Hydrochloric acid**

### Final Answer:
**A, C, B**