How are they finding class interval and width, I know this is a simple question but I’m losing my sanity over this. Please help. Any trick is also appreciated

The class intervals are shown as

lower limit upper limit

-9.19 <= -5.45

-5.45 <= -1.71 and so on

Cumulative relative interval is sum of all relative frequencies within a particular range of -1.71<= x < 2.03% out of the 12-year of returns.

You will find that there are 4 years which generate returns within mentioned range. In this case, each return is continuously (there is no returns with frequency more than one, because the data is continuous) and uniformly distributed (aka each has the same weight of probability). Therefore, cumulative distribution is 4/12 = 0.33 or 33%.

I’d strongly recommend you to confirm the result with the answer key, as I am candidate myself.