Answer:  Simple random sample
Explanation:
-  A simple random sample is an unbiased representation of a sample of a population. A subset of a statistical population is known as a simple random sample in which each member of the subset has an equal probability of being selected.
 
Given: In a small town there are 300 renters and 1000 homeowners. 100 renters and 100 homeowners are randomly selected to participate in a study involving annual household income. 
Here each renters and homeowners has the same probability of getting selected.
Therefore a simple random sampling is used in this study.