An oxidising agent is a substance that has the ability to cause oxidation in another substance. In an oxidation-reduction reaction, the substance being oxidised loses electrons, resulting in an increase in its oxidation number.

Option 1 states that an oxidising agent increases the oxidation number of an element in a given substance. This statement aligns with the definition of an oxidising agent and is correct. When an oxidising agent reacts with another substance, it accepts electrons, causing the oxidation of the substance by increasing its oxidation number.

Option 2 suggests that an oxidising agent decreases the oxidation number of an element in a given substance. This statement is incorrect because an oxidising agent causes the opposite effect by increasing the oxidation number.

Option 3 states that an oxidising agent is oxidised itself in an oxidation-reduction reaction. This statement is also correct as an oxidising agent accepts electrons, leading to its own reduction while causing oxidation in the other substance.

Option 4 mentions that an oxidising agent loses electrons in an oxidation-reduction reaction. This statement is not accurate as an oxidising agent gains electrons rather than losing them.

Overall, the correct answer is option 1, and it correctly explains that an oxidising agent increases the oxidation number of an