Option 1: A sub-tropical shrub refers to a plant that grows in regions with mild winters and hot summers. Coffee plants prefer warm climates and can tolerate mild winters, so this option is a possible choice.

Option 2: A warm temperate shrub refers to a plant that grows in regions with moderate summers and mild winters. Since coffee plants prefer hotter climates, this option is less likely to be correct.

Option 3: A tropical shrub refers to a plant that grows in tropical regions characterized by high temperatures and humidity. Coffee plants are native to tropical regions, making this option the correct choice.

Option 4: A cool temperate shrub refers to a plant that grows in regions with cool summers and cold winters. Coffee plants do not thrive in cool temperatures, so this option is unlikely to be correct.

Therefore, the correct answer is Option 3, as coffee is a tropical shrub.