Sodium is a highly reactive metal. It reacts vigorously with oxygen in air and also reacts violently with moisture/water, producing heat and hydrogen gas which can catch fire:
$$2\text{Na(s)} + 2\text{H}_2\text{O(l)} \rightarrow 2\text{NaOH(aq)} + \text{H}_2\text{(g)} + \text{heat}$$
Kerosene oil does not react with sodium. It covers the metal completely, preventing contact with atmospheric oxygen and moisture. This protects sodium from accidental fires and unwanted reactions.
Source: Chapter 3, Section 3.2.1 and 3.2.2
---
The examiner expects two key points: (1) sodium's high reactivity with both air (oxygen) and water, and (2) kerosene acts as an inert barrier preventing this contact. Writing the reaction with water earns full credit for a 2-mark question. Do not just say "it is reactive" — explain what it reacts with and why kerosene works (it is inert/does not react with sodium).