The valency of an element refers to its combining capacity with other elements to form compounds. In the question, we are asked about the valency of phosphorus.

Option 1 states that the valency of phosphorus is 2 or 3. This means that phosphorus can either form compounds by gaining or losing 2 electrons or by gaining or losing 3 electrons. However, this statement is incorrect as the valency of phosphorus is not commonly observed to be 2.

Option 2 states that the valency of phosphorus is 3 or 4. This means that phosphorus can either form compounds by gaining or losing 3 electrons or by gaining or losing 4 electrons. While phosphorus can form compounds with a valency of 3 in some cases, its common valency is 5. Therefore, this statement is partially correct but not completely accurate.

Option 3 states that the valency of phosphorus is 4 or 5. This means that phosphorus can either form compounds by gaining or losing 4 electrons or by gaining or losing 5 electrons. This statement is correct as the valency of phosphorus is commonly observed to be 5.

Option 4 states that the valency of phosphorus is 3 or