Concept:
This question examines the correct use of verb tenses based on time indicators present in a sentence.
The phrase “just now” indicates that the action has been completed very recently. In standard grammatical usage, recently completed actions that have a connection with the present are commonly expressed using the Present Perfect Tense.
The structure of the Present Perfect Tense is:
\[
\text{Subject} + \text{has/have} + \text{Past Participle (V3)}
\]
Step 1: Analyzing the subject.
The subject of the sentence is:
\[
\text{He}
\]
Since “He” is a third-person singular subject, the appropriate auxiliary verb is:
\[
\text{has}
\]
Therefore, any correct answer should contain the auxiliary “has.”
Step 2: Evaluating each option.
• had finished
This is Past Perfect Tense.
Past Perfect is generally used when one past action occurred before another past action.
No second past action is given in the sentence.
Hence, this option is incorrect.
• has finished
This is Present Perfect Tense.
It correctly uses the singular auxiliary “has” and the past participle “finished.”
It also expresses a recently completed action.
Therefore, this option is correct.
• will have finished
This is Future Perfect Tense.
It refers to an action that will be completed before a future point of time.
The sentence clearly refers to a recently completed action, not a future one.
Hence, this option is incorrect.
• finishes
This is Simple Present Tense.
Simple Present is generally used for habits, routines, or universal truths.
It does not express a recently completed action.
Hence, this option is incorrect.
Step 3: Constructing the correct sentence.
Substituting the correct option into the sentence gives:
\[
\text{He has finished the letter just now.}
\]
This sentence correctly expresses a recently completed action.
Therefore,
\[
\boxed{\text{has finished}}
\]
is the correct answer.