Select the most appropriate synonym to substitute the underlined word.

The weather forecast mentioned that there would be a cloud burst this afternoon.

The correct answer is rainstorm.

Let us analyze the meaning of the underlined term and the given options to find the most appropriate synonym:

1. Cloudburst: A cloudburst is a sudden, very heavy rainfall, which is often short-lived and can sometimes lead to flash floods. It is essentially an intense and sudden downpour or storm characterized by heavy rain.

2. Rainstorm: A rainstorm is a storm characterized by heavy rain, which directly corresponds to the sudden, torrential rain of a cloudburst. Therefore, "rainstorm" is the most appropriate synonym for "cloud burst".

Let's look at why the other options are incorrect:
- Sandstorm: A strong wind carrying clouds of sand, typically in a desert. This does not involve rainfall.
- Famine: An extreme scarcity of food, which is unrelated to weather precipitation.
- Snowfall: The fall of snow, which is frozen precipitation, whereas a cloudburst refers specifically to liquid rain.

Thus, the most suitable synonym to substitute "cloud burst" is rainstorm.