How are US amateurs restricted in segments of bands where the Amateur Radio Service is secondary?

Answer: A. U.S. amateurs may find non-amateur stations in those segments, and must avoid interfering with them

Where amateurs are secondary users, U.S. amateurs may find non-amateur stations in those segments and must avoid interfering with them, as FCC rule 97.303 explains. Primary users have priority. Secondary status does not ban international or digital contacts.

Tip: Secondary means 'share politely and do not interfere with the primary users'.

FCC rule reference: Part 97.303