The empirical formula of an ionic compound is the simplest ratio of the ions present in the compound. In this case, the unit cell of the compound consists of 'A' ions at the corners of a cube and 'B' ions on the centers of the faces of the cube.

Understanding the Unit Cell:
- The unit cell is the smallest repeating unit of a crystal lattice.
- In this case, the unit cell is a cube.
- Each corner of the cube has an 'A' ion, and each face of the cube has a 'B' ion at its center.

Finding the Empirical Formula:
To find the empirical formula, we need to determine the ratio of 'A' ions to 'B' ions in the unit cell.

'A' Ions:
- There are eight corners of the cube, and each corner has one 'A' ion.
- Therefore, the total number of 'A' ions in the unit cell is 8 x 1 = 8.

'B' Ions:
- There are six faces of the cube, and each face has one 'B' ion at its center.
- Therefore, the total number of 'B' ions in the unit cell is 6 x 1 = 6.

Ratio of 'A' ions to 'B' ions:
- The ratio of 'A' ions to 'B' ions in the unit cell is 8:6.
- However, we need to simplify this ratio to its simplest form.
- Dividing both numbers by their greatest common divisor, we get 8 ÷ 2 = 4 and 6 ÷ 2 = 3.
- Therefore, the simplest ratio of 'A' ions to 'B' ions is 4:3.

Empirical Formula:
Based on the above ratio, the empirical formula of this compound is A4B3. Therefore, the correct answer is d) A3B.

Summary:
- The empirical formula of an ionic compound represents the simplest ratio of ions in the compound.
- In this case, the unit cell consists of 'A' ions at the corners of a cube and 'B' ions on the centers of the faces of the cube.
- By determining the number of 'A' ions and 'B' ions in the unit cell, we can find the ratio between them.
- Simplifying this ratio gives us the empirical formula, which is A4B3 in this case.