State why the individuals in this group have different pulse rates before exercise.

Pulse rate is the number of times a person's heart beats in one minute. They depend on the fitness level of a person and also the age of a person.
People who are on medication, obesity, alcohol users, athleticism they produce pulse whereby a pulse is an effective way to measure heart rate.

Before exercise, a normal person i.e man and woman should have pulse rate 60-100 bpm(beats per minute. For teens who are at age of 10-20 years should have a pulse rate of 60-100bpm and children between 3-9 years should have 70-130bpm while infants of one day to two years of age should have a pulse rate 70-190bpm. Athletes may have as low as 40bpm.

Icant be totally sure but i would say with confidence that it is example #1 about cells.