When it requires de-emphasis, non-essential material in a sentence is set off by a:

b. pair of parentheses.
c. comma
d. colon
Hi Marilyn, welcome to the forums..

I would say 'brackets' (that's what I tend to use)
Either a pair of parentheses or a pair of commas. It depends on HOW non-essential the information is. The parentheses would indicate that it is even less essential than something you might put between two commas.
The other two are incorrect. You can't set off non-essential material with a period or a colon.
Students: We have free audio pronunciation exercises.
b.pair of parentheses
I'd argue your goals are in conflict. If de-emphasis is what you want, use commas. Brackets tend to emphasize their contents simply because they are less common. But brackets also suggest greater separation. So if non-essentiality is your goal, brackets say it most clearly.
For what it's worth, I disagree with the above comment.

There are several ways to set off pathenthetical elements. My understanding is that the use of brackets (like this) serves to DE-emphasise; the use of commas, like this, has neutral emphasis; and the use of dashes - like this - has positive emphasis.

But then - that's punctuation for you. It has rules of its own.
Teachers: We supply a list of EFL job vacancies
I just took my exam in this subject and the correct answer was a pair of parentheses - according to my online exam anyway for my college course
The accident it occurred four years ago crippled him.
AnonymousThe accident it occurred four years ago crippled him.

What's your question, please?
Site Hint: Check out our list of pronunciation videos.