Answer:
They are thinking about whether to support the interest of their constituents or what they think is best for the nation generally.
Step-by-step explanation:
When a congressperson votes as a trustee what are they thinking about is "whether to support the interest of their constituents or what they think is best for the nation generally."
This is because the United States Constitution did not state whether congress members should behave as a delegate (a representative that cast votes that favor his constituents) or as a trustee ( a representative that cast votes on what he believes is the best decision to support)