Under 14 CFR § 107.61(d), an applicant may demonstrate the aeronautical knowledge required for a remote pilot certificate by —

Why: 14 CFR § 107.61(d) allows an applicant to pass an initial knowledge test covering § 107.73, or, if the applicant holds a part 61 pilot certificate (other than student pilot) and meets § 61.56 flight-review currency, to instead complete training covering § 107.74.

• A notarized CFI letter is not one of the two paths § 107.61(d) recognizes; aeronautical knowledge must be shown through the knowledge test or § 107.74 training, not a third-party attestation.
• Part 107 does not require any manned-aircraft dual flight instruction to obtain a small UAS rating; the certificate is earned through knowledge demonstration, not flight training in a manned aircraft.