Final answer:
Implied consent is binding but difficult to prove because it depends on interpreting a person's actions or the context rather than a clear verbal or written agreement.
Step-by-step explanation:
The kind of consent that is binding but difficult to prove is C. implied consent. Implied consent occurs when a person's actions or the circumstances indicate that they have agreed to something, even though they may not have expressed it verbally or in writing. Unlike written or printed consent, which provide physical evidence of agreement, or oral consent, which might be witnessed, implied consent is often inferred from actions or the context, making it much harder to verify and prove in a legal setting.