Which business document is usually written in block style with the body text aligned along its left margin?
the
is usually written in block style with the body text aligned along the left margin.

The Business letter  is usually written in block style with the body text aligned along the left margin.

Explanation:

A business letter is a formal letter which written by an organization to another organization or clients for various purpose. The purpose may to advertise about the product, to complaint about the delivery, to inform about the new arrivals and discounts, to greet the client on special occasions, to seek information from the client, to apologize for delivering defective item and so on.

So the letters are always aligned left except for the header. Here the salutation and greetings are followed by comma which is similar to a British style.