Final answer:
The indicator that signifies a donor's eligibility to donate blood is a normal hematocrit level, specifically option D. Hematocrit level 42%. This level is within the normal range and indicates sufficient red blood cell volume for donation.
Step-by-step explanation:
The indicator that suggests a donor's eligibility to donate blood from the options provided is D. Hematocrit level 42%. A normal hematocrit level, which is the ratio of the volume of red blood cells to the total volume of blood, indicates healthy red blood cell levels and is essential for blood donation.
A hematocrit level of 42% is within the normal range for adults, which typically falls between 38.3% and 48.6% for men and 35.5% to 44.9% for women. Donating blood requires specific eligibility criteria to ensure both the safety of the donor and the effectiveness of the donated blood for the recipient.
In contrast, A. Pulse rate 40/min is below the normal resting range (60-100 beats per minute), and B. Diastolic blood pressure 100 mm Hg is on the upper limit but still within the allowable range for donation as long as systolic pressure is below 180 mm Hg.
Option C. Temperature 37.7 degree Celsius (99.8 degree Fahrenheit) may suggest a low-grade fever, which could disqualify someone from donating until they are healthy again.