I want to collectively smack every jabrony that ever posted what they think the 'right' answer is. No, its NOT 'leave it blank' you tools.
The 751 instruction, FIRST SENTENCE "This petition is used by a conditional resident"
That means that the CR IS THE PETITIONER.
Part 7 1.a/1.b/2. 2 is the correct answer if you are not going to select 1.a or 1.b. IE someone, other than an interpreter or lawyer, helped the CR (aka petitioner) fill out the form. Who? Most would say spouse, or if you got divorced, anyone can as long as they are willing to sign the form.
CR (aka petitioner) signs 6.a
Part 8 spouse has the same options. If you cannot chose 1.a or 1.b then 2 is the right answer.
Unless you had an interpreter (and one better be with you at the interview otherwise you cannot swear that you understand the questions they will ask since you didn't select 1.a in part 7) you can skip all of part 9.
Part 10 is filled out by the person you put in 2. of either part 7 or 8. Usually this would be the USC as they most likely are fluent in English. If not, then you better re-evaluate your answers. Showing up at a USCIS interview and not being fluent (like you said you were by checking 1.a in part 7 and 8 ) is gonna look super bad for you.
Its not hard. Places like VJ and other sites are full or jobronies that cannot read. Could you leave it blank? Sure. But expecting your USCIS officer to baby your spouse because they can't understand English is not going to end well for you.