Actually, with most of the examples, only the definite article would be used, since, together with the correct preposition they define a specific position.
For example, the post office is on the left.
If you would say it is "on a left", it would not only sound strange, it would imply that the post office is somewhere on a street to my left. ("A left" meaning a left turn, not "the left side of something"!)
You could say you are on "a second floor" somewhere, but this usage is very rare. Regardless of which specific building you are in, you will normally say you are on the nth floor.
Likewise, a flight of stairs only has one top, so you are at the top of the stairs. (Not "a" stairs, since stairs are plural anyway.)
With farm, you would use "the" to indicate a specific farm, but if you want to say it could be any farm, you use the indefinite article.
The only other places where you could use "a" would be the two corners. Being on the corner or being on a corner can both be used, but usually you would say "I am on the corner of a street."
In a corner and in the corner have different meanings. If you are in the corner, you are literally in the corner-area of a room. If you are in a corner, this will be understood figuratively, as being in a difficult position.
A long story short: most of these expressions are idiomatic and always used with the definite article. The one real exception is indeed the farm, where neither "a farm" or "the farm" have any idiomatic meaning.
It is a bit confusing that in the answers they chose to use the indefinite article, when the definite article could be used as well.
(Maybe they originally tried to go fro "in the country", which has an idiomatic meaning of "in a rural area", as opposed to "in a country", which means "inside the borders of a state")