Final answer:
An unconscious patient whose life is in danger can receive invasive medical treatment without signed consent due to the principle of implied consent in emergencies.
Step-by-step explanation:
The individual who can receive invasive medical treatment without a signed consent form is A. An unconscious patient whose life is in danger. In such emergency situations, medical professionals act under the assumption that the patient would consent to treatment if they could, following the principle of implied consent to protect the patient's life. This is distinct from other scenarios, where consent must be adequately informed and voluntarily given, and can involve forms like advance directives, DNR orders, or consent given by parents or legal guardians for minors.