Moving prepositions before relative pronouns in formal contexts
Q1. The man ___ whom I spoke was helpful
TO WHOM is formal. 'Who I spoke to' is informal. Both are correct. Register differs.
Q2. The colleague ___ whom I worked has left
WITH WHOM is formal. 'Who I worked with' is natural speech. Departure occurred.
Q3. The cause ___ which she fought was just
FOR WHICH is formal. Justice was served. Formality adds gravitas.
Q4. The house ___ which he lives is old
IN WHICH is formal. 'Where he lives' is simpler. Age is confirmed.
Q5. Which is most formal?
TO WHOM is most formal. Speech adapts to context. All are understandable.