Dear student,
a) We can make estimates for the total number of stars, but there isn’t a firm, solid, 100% answer.
Approximately we can say that there are 100 billion of stars in our galaxy.
b) There are approximately one hundred a billion galaxies as well in our universe.
c) The number of the universe is still an unknown information. So we really can't say anything about the approximate value.
d) Multiverses are also unknown quantity yet. Scientists have raised the provocative notion that the answer may depend on the human brain.
Regards