Which corrects any error in the use of apostrophes in the sentence?
my little sister sometimes writes her letter b's backwards.
a. writes'
b. backward's
c. there is no error in this sentence.
d. bs

The answer is C.
There is no error in the article.
B's is used as a plural and when a single character is punctuated, we as an apostrophe folled by 's' to avoid any easy confusion by the reader.
I hope this helped.