What motor vehicles may enter an environment zone that has this sign?

You answered correctly. In Germany, an environment zone (Umweltzone) with this sign means only vehicles with a green particulate emissions sticker are allowed to enter. Motorcycles are also allowed to enter, even without a sticker, as they are exempt from these regulations. However, simply meeting the requirements for a green sticker is not enough; the sticker must be displayed on the vehicle. This is important to remember for future questions: always check both the emissions requirements and the need for a visible sticker, except in the case of motorcycles.