Which statement is true about amateur radio equipment?

Amateur radio equipment can indeed include modifications to enhance performance, which is a key aspect of the hobby. Amateur radio operators are often encouraged to build and modify their own equipment as part of their learning process and to tailor their stations to their specific needs or interests. This flexibility allows operators to experiment with different configurations, improve their equipment's efficiency, and adapt it for various operating conditions.

The context of amateur radio is centered on experimentation and self-education, and part of that process involves making changes to existing commercial gear or building devices from scratch. This culture of modification is integral to the amateur radio community, allowing for a diverse range of capabilities and innovations.

The other statements do not align with the principles of amateur radio. Commercial manufacturing is not a requirement, as homemade equipment is widely accepted and frequently built by operators. Additionally, operating only on manufacturer-specified frequencies contradicts the amateur radio regulations, which allow operators to use specified bands and frequencies as long as they adhere to the licensing rules set forth by the governing bodies.