Is the blood pressure of a group of students discrete or continuous?
Blood pressure itself is continuous, but the measurement of it would appear to be discrete because of the limit on accuracy which can be recorded.
Any physical quantity which has to be measured has to be continuous. For example, as a girl grows taller, her height will be every possible measurement up to her measured height.
Her height cannot "jump" from 1.25 metres to 1.3 metres.
In the same way, mass, time, distance, speed, pressure etc are quantities which increase continuously rather than in jumps.
The problem is in the measurement. Practicality dictates that we have to limit measurements to a certain accuracy, we cannot measure everything to 10 or more decimal places!
For this reason, continuous data is sorted into groups or classes with well defined boundaries.