At minimum, other users on Quora, per the letter of the policy.
Quora’s answer to What is Quora’s “Be Nice, Be Respectful” policy?
Increasingly, it appears from Moderation decisions, also public figures who are not already users on Quora:
Habib Fanny: BNBR against public figures not on Quora by Nick Nicholas on The Insurgency
See in particular the comments to that report.
See also Quora Moderation — Election Season PSA by Marc Bodnick on The Quora Moderation Blog—which first announced that not being nice and respectful to political candidates was unacceptable. The rationale for this being that abusive criticism of political leaders makes Quora an unwelcoming space for those leaders’ supporters.