In this question, we are asked to identify which of the given options are considered `rain clouds`.

Option 1: Cirrus - Cirrus clouds are high-altitude clouds composed of ice crystals and generally do not produce precipitation. Therefore, it is not a rain cloud.

Option 2: Nimbostratus - Nimbostratus clouds are thick, dark gray clouds that indicate steady precipitation. They are often associated with rain or snow, making them a rain cloud.

Option 3: Cumulonimbus - Cumulonimbus clouds are tall, dense clouds that are capable of producing heavy rain, thunderstorms, and even tornadoes. They are commonly known as thunderstorm clouds and are a type of rain cloud.

Option 4: Altocumulus - Altocumulus clouds are mid-level clouds that do not typically produce precipitation. Therefore, it is not a rain cloud.

Based on the explanations above, the correct answer is option 2: Nimbostratus and option 3: Cumulonimbus only.