Concept:
Phrasal verbs often have meanings that cannot be understood merely by combining the meanings of the individual words.
The phrase needed in this sentence must express the idea of a problem appearing, arising, occurring, or presenting itself.
The sentence advises a person not to think about the solution before the problem actually arises.
Step 1: Analyzing the context carefully.
The sentence reads:
\[
\text{Don't worry about the solution until the problem _ _ _ _ .}
\]
This means the problem has not yet appeared.
The missing phrasal verb should therefore mean:
• to arise,
• to occur,
• to appear,
• to present itself.
Step 2: Examining each option.
• comes off
This means succeeds, happens successfully, or becomes detached.
It does not mean “arises.”
Hence, it is incorrect.
• comes in
This means enters, arrives, or becomes fashionable.
It does not suit the context of a problem appearing.
Hence, it is incorrect.
• comes up
The phrasal verb come up means to arise, occur, or present itself unexpectedly.
This matches the intended meaning perfectly.
Hence, it is correct.
• comes through
This generally means survives a difficult situation or succeeds after difficulties.
It does not mean “arises.”
Hence, it is incorrect.
Step 3: Constructing the final sentence.
Substituting the correct option gives:
\[
\text{Don't worry about the solution until the problem comes up.}
\]
This sentence conveys the intended meaning accurately.
Therefore,
\[
\boxed{\text{comes up}}
\]
is the correct answer.