Question

Choose the correct possessive form.

If there were more than one Secretary of State, and they both lived in this house, this would be the _____ house.

Secretary of State's
Secretaries' of State
Secretary of States'
Secretaries of State's

Answer 1

old question but I am answering so this can be closed

Secretary of State is the complete title, and therefore the plural is made by adding the apostrophe + s at the end --> Secretary of State's

now, for the plural form, simply make the word "secretary" plural, without changing anything else.