In a laboratory population of 80 fruit flies, 8 died in a week, then…

The correct answer is Option (3) → 0.1 individuals per fruit fly per week

Death rate = 8 ÷ 80 = 0.1 individual per fruit fly per week.

Option (1) 0.3 individuals per fruit fly per week — Incorrect. Death rate is deaths ÷ individuals: 8 ÷ 80 = 0.1. A rate of 0.3 would need 24 deaths in the 80 flies.

Option (2) 0.2 individuals per fruit fly per week — Incorrect. 8 ÷ 80 gives 0.1, not 0.2. A rate of 0.2 would need 16 deaths.

Option (3) 0.1 individuals per fruit fly per week — Correct. Death rate is the number of deaths per individual over a period. Here 8 of 80 flies die in a week, so the death rate = 8 ÷ 80 = 0.1 individuals per fruit fly per week.

Option (4) 0.01 individuals per fruit fly per week — Incorrect. 8 ÷ 80 is 0.1, not 0.01. A rate of 0.01 would mean fewer than one death in 80 flies.

NCERT Extract:
Population Attributes – “… If in a pond there were 20 lotus plants last year and through reproduction 8 new plants are added, taking the current population to 28, we calculate the birth rate as 8/20 = 0.4 offspring per lotus per year. If 4 individuals in a laboratory population of 40 fruitflies died during a specified time interval, say a week, the death rate in the population during that period is 4/40 = 0.1 individuals per fruitfly per week.”