First Principles Thinking: what counts as 'written consent'
C is correct. Handbook guidance expressly defines 'written consent' to include any form of communication that can be documented and retrieved, with email given as the canonical example. Okafor disclosed the nature, approximate amount, and duration of the arrangement in advance and received a retrievable written approval, which is exactly what the standard contemplates.
A applies a paper-only formalism that the standard does not impose. Email is a specifically approved channel.
B conflates employer consent with party acknowledgment. Best practice is for the paying party to confirm the details as well, but where the employer has approved a disclosed arrangement, acceptance does not become a violation merely because a separate third-party signature has not yet been obtained.