Final answer:
The sampling method that would most likely have the smallest margin of error is both a and c.
Step-by-step explanation:
The sampling method that would most likely have the smallest margin of error is d) Both a and c.
In option a, flipping a coin 100 times, the margin of error would be small because the sample size is large enough to provide a good estimate of the proportion of heads.
In option c, sampling 10 adults, the margin of error would also be small because the sample size is small and specific, making it easier to get a representative sample.