Rabbits were introduced to Australia in the 1800s. They rapidly overpopulated because they had few natural predators in the area. To control their population, scientists introduced a rabbit-specific virus into the population, and their numbers greatly decreased. However, after several generations, the rabbit population began to increase again. Which statement explains the new increase in the number of rabbits?

This is an incomplete question. In the following paragraph you can find the complete statement:

Rabbits were introduced to Australia in the 1800s. They rapidly overpopulated because they had few natural predators in the area. To control their population, scientists introduced a rabbit-specific virus into the population, and their numbers greatly decreased. However, after several generations, the rabbit population began to increase again. Which statement explains the new increase in the number of rabbits?

a. The rabbits interbred with the native rabbit species.

b. Some rabbits learn to survive even though they were sick.

c. Some rabbits have natural inmunity to the virus.

d. The rabbits were able to have more offspring by changing their reproductive cycles.

Explanation:

As the introduction to the question states, rabbits were introduced to Australia in the 1800s and they rapidly increase their populations because has only few natural predators in the area. The enormous population developed started to cause some problems so scientits decided to introduced a biological agent, particularly a virus, to control the expansion of the species. At first, the number of individuals decreased, but then, after several generations, they started to increase again. The explanation is that some rabbits have natural inmunity to the virus so, natural selection operated through time, selected the resistant individuals, they reproduced and population rised.

In the original habitat (not in Australia) of the rabbit, it coexist with the virus so, it is not weird that the original population had resistance. This resistance was transmited by the introduced rabbits, persisted in some individuals and  later resulted in an adaptive adventage when the virus was introduced in Australia.

To summarize, the correct option is c. Some rabbits have natural inmunity to the virus.    

your answer is "the 1950's"