An opinion poll in california uses random digit dialing to choose telephone numbers at random. numbers are selected separately within each california area code. the size of the sample in each area code is proportional to the population living there. (a) what is the name for this kind of sampling design?

The answer is stratified random sample.  Stratified random sampling is a technique for sampling that includes the division of a populace into littler gatherings known as strata - which depend on individuals' shared qualities or attributes. To do this, you have to first separate the populace into groups of people that are comparative somehow. At that point pick a basic random sample from each and join to frame a full example.