the main form (like primitive form) of that setnence in translated is a.얼마나 빨라? b.얼마나 빠른가? but this expression usually used when you talk with same age or youngers (not just age, sometimes ruled by social level or etc... ) and there is little difference between sentence 'a' and 'b' 'a' is used with freindly and 'b' is used with formal the normal form of that sentence is just like you said A.얼마나 빨라요? B.얼마나 빠른가요? these two sentence is from primitive form and just added '요' and '요' makes a sentence polite which is why we use it as normal and those two sentence keep the character(nuance) of primitive form so sentence 'B' is more polite than 'A' so as a answer you can use a,b,A,B all as you want there are some other forms are existing but you can ignore them at now on I wish you good luck in studying Korean