Concept:
An idiom is a group of words whose meaning cannot be understood from the literal meanings of the individual words.
The idiom “call it a day” is widely used in everyday English to indicate stopping an activity, especially work, because enough has been done or because one is tired.
Step 1: Understanding the idiom.
The expression:
\[
\text{call it a day}
\]
does not literally mean announcing that it is daytime.
Instead, it figuratively means:
• to stop working,
• to finish an activity,
• to end the day's efforts.
Step 2: Using the context to determine the meaning.
The sentence states that the person has been working since morning.
By 5 p.m., he decides to stop working.
Therefore, the idiom clearly indicates ending the work for the day.
Step 3: Evaluating the options.
• continued to do his work
This is opposite to the meaning of the idiom.
Hence, incorrect.
• decided to stop his work
This exactly matches the meaning of the idiom.
Hence, correct.
• decided to begin his work again
This conveys restarting work rather than stopping it.
Hence, incorrect.
• thought there was still sunshine
This is merely a literal interpretation of the word “day” and does not reflect the idiomatic meaning.
Hence, incorrect.
Step 4: Final conclusion.
The idiom means ending one's work or activity.
Therefore,
\[
\boxed{\text{decided to stop his work}}
\]
is the correct answer.