Spouse visa - send in sponsor's passport/BRP?

[FreshFromTheGrave]

Hi all! Quick easy question for once (I hope).

When applying for a spouse visa should the sponsor send in their passport/BRP card with the application? I have a South African passport and a separate BRP card for my ILR. Do I:

a) Send a normal non-certified copy of passport ID page and front and back of BRP card? (Presuming they just need to run the number on the card through their system to check if it's legit)
b) Send in actual passport and BRP card?
c) Send in BRP card only to avoid confusion with expired visas in an irrelevant passport?

A document on gov.uk seems to suggest a) but some forum threads seem to suggest otherwise. Always good to check, thanks everyone!