Answer
Option C is correct.
She could use a random number generator with each day assigned one number.
Step-by-step explanation
The thing about sampling of this matter is that it must be very random. And for random sampling, all the possible outcomes must have the same chance of happening.
Option A cannot be correct because a six-sided cube has only spaces for 6 days. This leaves out one of the 7 days.
Option B cannot be correct too as a frequency table would mean that that she recorded the days that she has picked to start the vacation a couple of times. This isn't random enough as not all of the days will evidently have the same chance of being picked.
Option C seems to be the most correct option because it is the most random of the techniques. She assigns different numbers to each of the 7 days, one for each day. Then she uses the random number generator to pick one number and subsequently pick the day that corresponds to that number.
Option D cannot be correct. This is because a coin only has two possible outcomes, and we have 7 outcomes to test for.
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