A substitute product of most concern for a cable TV distributor is:

A substitute product that poses a significant concern for a cable TV distributor is satellite TV. This is because satellite TV directly competes with cable television by offering similar services such as a wide range of channels, high-definition programming, and additional features like bundling services (e.g., internet and phone) at competitive prices. Customers may be attracted to satellite TV for various reasons, including pricing, programming offerings, or the ability to access channels without the constraints of a physical cable connection.

While broadcast TV, primarily using antenna signals, offers free-to-air content, it does not provide the extensive channel selection or premium content available through cable or satellite services. Satellite radio, while an alternative form of entertainment, does not replace traditional TV services, making it less relevant in this context. The internet, while it has become a platform for video streaming services like Netflix and Hulu, represents a broader category than just a direct substitute for cable but introduces competition through different types of content consumption rather than traditional cable or satellite offerings.

Therefore, among the given choices, satellite TV is the most direct competitor and substitute for cable TV services, making it the primary concern for a cable TV distributor.