Are "that", "who" and "which" in these sentences necessary or optional?

• I think blue is the colour that optional __________ I like best.
• Simon, who necessary__________ is 25, got married last week.
• My car has a mechanical problem that /which necessary __________ is expensive to repair.
• He left the keys that optional __________ I gave him on the table.
• The carpenter who necessary __________ made my book shelf is a rich man.
• He took a plane that necessary __________ left from London.
• The money which optional __________ you borrowed was not really necessary.
• James is the most honest person that optional __________ I know.
• The book that / which optional _____________ I found very interesting was the French one.
• They are the people who necessary _____________ live next door.