Answer:
a. The black dog barked loudly as he passed on the sidewalk the mailman.
Explanation:
Rmemeber that fluency in writting refers to the writting in which words como out naturally, with a constant flow and rhythm. In other words, by reading a fluent sentences, the reader won't have any trouble undestanding it easily.
In this case option A. isn't very fluent as it drives the reader to confusion.
"The black dog barked loudly as he passed on the sidewalk the mailman."
The placing of the word "mailman" makes it feel akward an unnatural. it is kind of repeating itself, being redundant.
It would be better to put it like this:
"The black dog barked loudly as the mailman passed on the sidewalk."
