State the name of the sampling method used in each of the following scenarios.
(a) When a biologist takes a sample from one location in a lake rather than randomly sampling various locations in the lake.(b)A researcher is interested in opinions about stem cell research. A random sample of U.S. residents is selected, and the respondents are asked questions about stem cell research.
(c) A researcher would like to investigate the spread of some kind of cow disease. He selected randomly four farms out of 200 farms and surveyed all the cows in those selected farms.
(d) A public opinion poll is conducted in Virginia to determine whether registered voters in the state approve of a measure to ban smoking in all indoor restaurants in the interest of public health. The researchers select a simple random sample of 50 registered voters from each county in the state and ask whether they approve or disapprove of the measure.

what this biologist does is to take a sample from the lake in a non random fashion. It is a non probability sampling method. And it doesn't involve selecting participants randomly.

2. simple random sampling

the question says that the respondents who are US residents are randomly selected. Each of the resident has equal probability of being chosen

3. cluster sampling method:

this researcher groups the full population into 200 farms and then he picks 4 clusters randomly then he surveyed all cows in the 4 selected farms

4. stratified sampling method:

The researcher shares the full voter population into several counties which are regarded as stratas. From where a simple random sampling of 50 registered voters from each of the counties are selected.