Which honorific is used to address a married woman in English-speaking countries?

In the modern era, there's no reason to have different terms of address for women based on marital status, when there are no such terms for men. No argument can disprove that this is intended to define a woman's status as dependent on male approval in some way. So: Mr. for men, Ms. for women. Easy and fair, and respectful in both cases.

TRUE CANADIAN Laurel, Canada is in America, North America

Missing in this explanation is that "Mistress" was a feminine cognate for the word "Master", as in: the head of a household or owner of an estate. Thus, an unmarried woman could have been a "mistress" if she had maids or servants to supervise. No doubt these words were then also applied to folks who had no servants, too.

It was quite easy

TRUE CANADIAN Laurel, Calm down, mellow, no reason for this. As the explanation stated American English or British English (which would be, as you stated, The Queen's English) is equally represented & acknowledged here. Yes, there are many places in the universe that English is spoken, however, the Q/A/E with this specifically singled out the two as American English & British English.

"Mistress" used to be a very honourable title, but now it has changed its meaning.

I don’t know where you got this phrase American English. We in Canada also speak English and we have called it the Queen’s English. As I have said, there are more countries in the world than the USA. We stand alone, we spell certain words differently and we pronounce certain words differently, so PLEASE don’t even say that the other English is American, it is an insult to us and many other countries who speak English.

Mrs derived from the contraction Mr's, i.e. belongs to Mr.