Can anyone help me with this probability question?

I think it works like this.

1 has been eliminated by using census data to get a balanced sample.

2 could be right, but I don't think it's a serious bias. Just choosing one night out of the week is a bit limiting.

3 doesn't seem right. 8 weeks is short, but not that short.

4 is my choice for the answer. I don't think people have an accurate idea of how much sleep they get. It's a complicated thing. I sleep with a Fitbit that tells me what my sleep was like, breaking it up into Awake, REM sleep, Light Sleep, and Deep Sleep. That's certainly not as accurate as what a sleep lab would give you, but it's way better than my trying to get an estimate by myself.

I find questions like this tough, since it seems like a matter of opinion which is MOST likely to cause bias.

But for what it's worth, there's my reasoning.