It depends on what visa you are applying for and which country you are applying in, because it could be any of the three options.
If you are applying for an initial fiance or spousal visa from outside the UK:
- the sponsor provides a copy of the photo page of their passport only. Ideally it should be certified, but a photocopy seems to be accepted okay
If you are applying for an FLR(M) or ILR visa extension from inside the UK and are applying by post:
- the sponsor provides a full photocopy of EVERY page of their passport, including blank pages. It does not need to be certified
If you are applying for an FLR(M) or ILR visa extension from inside the UK and are applying in person at a Premium Service Centre:
- the sponsor attends the appointment in person, and so provides their actual passport