Which type of cloud is mostly likely present on gray, rainy day

Nimbostratus clouds.

Explanation:

Nimbostratus clouds are the type of clouds that are mostly likely present on gray, rainy day. These clouds produce continuous rain, snow and there is no thundering or lightening either. They are low level clouds that result in moderate rainfall. These clouds contain water droplets and may also contain snow or ice when temperature is cold enough. They often bring precipitation and their depth is over 20000 feet i. e. from ground level to high levels. They occur along a warm front and are formed in the middle level of the troposphere further spreading into the low and high levels.

Nimbostratus Clouds.