Federalists believed in the separation of powers because they wanted individuals to have separate rights. states to have separate rights. the central government to avoid monarchy. the nation to elect a strong executive.

Separation of powers - Prevents the government from becoming too powerful

This has to do with national government, so the first two choices are eliminated (they don't make sense anyway). Separation of powers limits the government, so the last option is eliminated.

This leaves avoiding monarchy as the only choice.

The Federalists believed in the separation of powers because they wanted the states to keep the laws they already had. They also wanted limited Federal government, as well as checks and balances.