The valency of A in A2B3 is 3.

The chemical formula A2B3 indicates that the compound consists of two atoms of element A and three atoms of element B. To determine the valency of A, we need to understand how the elements combine to form compounds.

1. What is Valency?
Valency is the combining capacity of an element. It indicates the number of electrons an atom can gain, lose, or share to form chemical bonds with other atoms.

2. Understanding the Chemical Formula A2B3
In the given compound, A2B3, the subscript 2 after A indicates that two atoms of element A are present, and the subscript 3 after B indicates that three atoms of element B are present.

3. Rule for Determining Valency
To determine the valency of an element in a compound, we need to consider the ratio of the combining atoms. In this case, the ratio of A to B is 2:3.

4. Applying the Rule to Determine Valency of A
The ratio of 2:3 suggests that element A has a valency of 3. This means that each atom of A can combine with three atoms of B to form the compound.

5. Example
For example, let's consider the compound NaCl. The valency of Na is 1 because it combines with one atom of Cl to form the compound NaCl. Similarly, in the compound MgO, the valency of Mg is 2 because it combines with two atoms of O.

6. Conclusion
Based on the given chemical formula A2B3, the valency of A is 3. Each atom of A combines with three atoms of B to form the compound. Therefore, the correct answer is option (a) 3.

It will be 3 ,as valency is cross multiplied in chemical formula.