Please advise wich is correct ?

(a) With respect to your letter dated

or

(b) With reference to your letter dated
Oh, Darlin’ do you like chocolate or vanilla! It’s that simple. They’re both good! But if you want to be ever so cautious, then hunt around for this book. We use it all the time and it’s for people who need to be ‘fashionably correct’.

MLA Style Manual and Guide to Scholarly Publishing (2d ed)
i think its

refering to your letter OR

in refrence to your letter
Students: We have free audio pronunciation exercises.

with reference to your letter dated.