Why is the answer for this question TRUE?

"According to recent data about 21% of adult smokers used e-cigarettes in 2015, which is more than double the rate in 2016."

Your understanding of this statement is entirely correct, as in the first part they provide the information that "about 21% of adult smokers used e-cigarettes."

They follow this up with "which is more than double the rate in 2016."

The "which is" must be referring to the percentage of "adult smokers using e-cigarettes."

So the following "more than double the rate in 2016." Must mean that the number of adult smokers in 2015 IS MORE than double the rate in 2016. Meaning that in 2015 there were more than twice as many adult smokers using e-cigarettes.

NOW SCRATCH EVERYTHING I JUST SAID

The part of the statement "which is more than double the rate in 2016." could POSSIBLY mean that- OF WHICH the percentage of adult smokers in 2015, it is more than double the rate in 2016."

The "It" refers to the percentage, and the statement is saying the percentage is more than double in 2016.

IN 2016 IT IS MORE THAN DOUBLE.

This would mean that there are more than twice as many smokers in 2016.

IN CONCLUSION, The creators of this question worded it incorrectly, and any loss of points because of this question is not justified.

I believe the exam question has mistakes in it.

According to recent data about 21% of adult smokers used e-cigarettes in 2015, which is more than double the rate in 2016.

This statement is very strangely worded. Normally you'd talk about the current (or most recent) situation, then compare it to a previous (older) situation. That's how time works: it goes forwards.

I believe it contains a typo and the years have been accidentally swapped.

This is consistent with the question, which you now know to be true. It makes most sense as well.

1. In 2016 twice as many people used e-cigerattes as in the previous year. TRUE

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To me, the question is also incorrectly worded. You cannot say "twice as many did something as in the previous year". You can say "than in" or "compared to" the previous year.