The presence of cumulus clouds usually indicates:

The presence of cumulus clouds usually indicates unstable air, which is often characterized by vertical development and the potential for thermal updrafts. In an unstable atmosphere, warm air rises rapidly, leading to the formation of cumulus clouds. This rising air can create turbulent conditions, but it also typically allows for clearer visibility in the spaces between the clouds.

Cumulus clouds often indicate fair weather, especially when they appear fluffy and well-defined, suggesting that the atmosphere is buoyant and not overcast with persistent clouds. This contrast in visibility can be attributed to the nature of these clouds, which do not generally produce extensive precipitation or fog. Therefore, the clear visibility associated with unstable air and cumulus cloud formation aligns with the correct answer.