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 -

0.1 individual per fruitfly per week

The correct answer is Option (2) → 0.1 individual per fruitfly per week 

A population has certain attributes whereas, an individual organism does not. An individual may have births and deaths, but a population has birth rates and death rates. In a population these rates refer to per capita births and deaths. The rates, hence, expressed are change in numbers (increase or decrease) with respect to members of the population.

If 4 fruitflies in a laboratory population of 40 fruitflies die in a week then the death rate of fruit flies is 4/40 = 0.1 individuals per fruitfly per week.

Death rate = (Number of deaths) / (Total population)

In this case:

Death rate = 4 deaths / 40 fruit fly

Death rate = 0.1 deaths per fruit fly per week