Recently a manuscript from my Masters by research thesis has been accepted in a Q1 journal with Impact factor nearly 4. At the beginning of the manuscript submission, I requested my advisers to be the coauthors of my article but they denied my request. Is that ethically ok to publish the accepted manuscript?
There are a few different situations that could lead to this:
Most plausible case: Your supervisors determined that in accordance with the standards of your field, they do not count as coauthors.
Different fields have different customs concerning whether supervisors are assumed to be coauthors of papers resulting from the supervised work. If you worked independently and your supervisors agree, then being a single author on the resulting paper will always be reasonable. Since you mention asking them only at submission stage, it does sound as if their involvement is limited.
Your supervisors really ought to be coauthors by the standards of your field, but declined to give you an (unfair) advantage or similar.
This is problematic from an ethical perspective. Having another persons consent still doesn't allow an academic to present their ideas as ones own. A carefully worded acknowledgement might work.
Your supervisors declined because they are aware of flaws in your paper, and don't want to be associated with it.
While one would hope that peer review would spot problems, there is no guarantee that it does. If your supervisors are aware of any major flaws, it would be your ethical responsibility to find out and either fix the paper, or to withdraw it if it is unfixable.
The manusscript is sound, but your supervisors are under political pressure to not associate themselves with the results. (pointed out by nick012000 in the comments)
Probably no ethical concerns about you publishing here, unless the political pressure is there for very good reasons.
If your advisors do not want to be co-authors, I think it is fine. You do not have to feel any sort of ethical conundrum for that matter.
By the way, have you asked them why they do not want to be co-authors?
Maybe they are already well accomplished and well published and want you to be the sole author of the paper; which will be a good thing for you. If that's the case, then your advisers are godly.
While the other current answers are good, they don't directly answer the main question. Yes it is ethical for you to publish on your own. There should be no question about that. Actually, be thankful that they almost certainly think enough of you that they don't see the need or have the desire to share your glory. It is good that the answer of Arno raised the third point, but I think the tis very unlikely.
Ethics enters in to authorship questions when people are improperly included or excluded from authorship. But there is no question of "exclusion" here, since you are acting on their recommendation when publishing without them.
Yes, PK1995 implies that there is no ethical constraint.