Learning a foreign language, such as french or spanish, is based on your mastery of the basic rules of english grammar

which is the best way to revise sentence (13)?
a) the sentence is correct as written.
b) learning a foreign language such as french or spanish is based on your mastery of the basic rules of english grammar.
c) learning a foreign language such as french or spanish, is based on your mastery of the basic rules of english grammar.
d) learning a foreign language, such as french or spanish is based on your mastery of the basic rules of english grammar.