Answer:
b. We will send your order as soon as we have confirmed the sizes you need
Explanation:
The second sentence b) is more casual than the first sentence a) simply because the words used are part of everyday english language.
Despite that, the sentence in ordered in a manner that is both polite, and clear, which gives it a professional tone while at the same time being easy to understand, meaning that the sentence can easily achieve the goal of providing effective communication to the customer.
