Final answer:
The most significant finding in a patient experiencing an allergic reaction is a widespread rash.
Step-by-step explanation:
The MOST significant finding in a patient experiencing an allergic reaction is d) Widespread rash. A widespread rash is a prominent symptom of an allergic reaction and can be a sign of a systemic type I hypersensitivity reaction, such as anaphylaxis. Other symptoms mentioned in the options, such as hoarseness, headache, and abdominal cramps, can also be present in an allergic reaction, but a widespread rash is typically the most visually noticeable and significant.