As you wish mate. My E6B has to 2 entries for a pressure altitude question.
Pressure - Pressure is 992
Altitude - 968ft
My simple understanding is that if you are 968 ft and the pressure at 968 ft is 992 if you add 21mb to that then the pressure altitude is 1598 ft. You have added 630 feet by taking it to 1013 which gives you the pressure altitude. Pressure altitude being a common datum rather than QNH.
The question does not say anything about an altimeter it just asks what the pressure altitude is based on the figures given.
I just plugged in the 2 figures from the question and out popped an answer of 1598ft which was the same as my original old fashioned calculation and as answer A on the quiz.
I clearly do not understand how you are working it out. But please feel free to edify us.