statistics !!

If a particular batch of data is approximately normally distributed, we would find that approximately

a) 2 of every 3 observations would fall between +- 1 standard deviation around the mean.
b) 4 of every 5 observations would fall between +-1.28 standard deviation around the mean.
c) 19 of every 20 observations would fall between +-2 standard deviation around the mean.
d) All of above.

Which one the correct answer? Anyone can explain for me ??