Idiomatic meanings can be hard to find in a dictionary. Sometimes they are listed under one of the main words, sometimes they are listed as a full and separate entry.
For example, if you look up mind in Collins, you'll see idioms such as:
- blow someone's mind
- give someone a piece of one's mind
- make up one's mind
and if you look up track, you'll find:
- the right track
- the wrong track
- off the beaten track
In Collins, though, one-track mind is not listed – probably because one-track gets its own entry.
Macmillan lists it slightly differently, though; it has an entry for one-track mind.
Sometimes you'll find the meaning of the idiom within the definition of one of its key words; other times you'll find the idiom listed by itself. In the case of a test question, when you don't know what the right answer is, you might have to try several possibilities before you stumble across the right one.
OneLook is an especially good on-line resource for this, because it searches several dictionaries for a word or phrase. For example, when I enter one road mind into OneLook, it apologizes and informs me that no matches were found, but when I enter one track mind, it finds matches in four dictionaries.