It should be a.), all plant cells have a cell wall made of cellulose, which acts as exoskeleton and makes them so stable. However on a bigger level this plant cells can also form cellulose fibers, so that c.) is not completely false :)
6 is the mode since it appears the most, 3 times
300 digits/numbers are used.
150pages * 2(two sides to one page) = 300numbers
Hope this helps!!!
By pythagoras Theorem.
x² +12²=13²
=>x² +144 =169
=>x² =169-144=25
=>x=√25=5 unit.
Hence x=5 is your answer.
Hope it helps...
Regards,
Leukonov/Olegion.