Explanation:

There are 3 ingredients needed for a thunderstorm:

1. moisture (water vapor) in the air near the surface
2. instability - relatively large decrease of air temperature with height
3. lift - some process (such as a front) that forces air to rise

Thunderstorms can occur at any time of day or night, but they're most common in the afternoon because that's when the temperature near the surface is generally at its highest under conditions of sunny skies. That warming near the surface means the air is likely to be the most unstable during the day. It also means that it takes less effort to lift the air by whatever mechanism is doing so. Therefore, 2 of the 3 ingredients are at a maximum during the afternoon.