Which of the following is a way that zebra mussels can be spread into inland waters?

Zebra mussels can spread to inland waters predominantly through human activities and equipment, particularly via bilges and engine cooling systems of boats. When boats are not properly cleaned after leaving infested waters, adult zebra mussels or their larvae, known as veligers, can remain in the water within these systems. If a boat is then launched into a different body of water, it can introduce zebra mussels to that new ecosystem, facilitating their spread.

The other options involve activities that are less direct or effective in spreading zebra mussels. While birds can physically transport zebra mussels, they are not a primary means of spreading these species to inland waters. Recreational boats, while they can spread zebra mussels, does not encompass the broader range of equipment that may contain them, such as those mentioned in the correct answer. Intentional stocking of fish does not contribute to zebra mussel spread, as they are not transported via fish. Instead, proper boat and equipment maintenance is crucial to prevent the spread of this invasive species.