It doesn't really make sense in English. I'd say "the gentleman" or something similar.
One thing that's really important to remember is that terms of address work slightly differently in Spanish. There are two types of address: direct and indirect. Direct is when you're speaking directly to the person ("Hi, Mr. Arroyo!"), and indirect is when you're referencing them to someone else or speaking about them in the third person ("Mr. Arroyo is my neighbor.").
In English you can use "Mr. Arroyo" for both, but in Spanish you can't. Indirect addresses need to be proceeded by an article (e.g. el, la). "Sr. Arroyo es mi vecino," should instead be "El Sr. Arroyo es mi vecino."
Ah yes, I think I hear what you mean. It sounds like something out of The Little Rascals "Hey Mister, we need some lumber!"
I was more thinging along the lines of an adult saying it when someone bumps into you, or skips you in a que
\"Oi, Mister, you better watch youself!"
You wouldn't use "Sir" in that way. It you want you can use "Sir" scarcastically/ ironically, such as
\"Well you Sir, are an ass!".
I think that ""El señor,"should be accepted as "mister," without the "the." It is confusing to me since, when referring to a person in the Third Person, (E.g. "Mr Perez eats with us,") you don't say "THE Mr Perez eats with us," in English, but in Spanish, you DO. ("EL Sr. Perez come con nosotros.") The sentence given has no context, so I think the permitted answers should be a bit more lenient.
Hahaha, it even says "the man" as the translation given by the website. Which honestly I think is a good answer (and the woman for la senora), simply because if you were to recreate the situation in which someone would say, "el senor" then in english they would say "the man probably half the time, and the gentleman the other" !