Select the most appropriate option that can substitute the highlighted segment in the given sentence. If there is no need to substitute it, select ‘No replacement required’ .

It is essential that every athlete completes their registration before the deadline.

The correct option is "complete their registration before the deadline".

Step-by-Step Explanation:
1. Identify the grammatical structure: The sentence begins with the construction "It is essential that...", which expresses necessity, importance, or urgency. In English, adjectives of urgency or importance (such as essential, important, necessary, crucial, mandatory) followed by a "that"-clause require the verb in the clause to be in the subjunctive mood.
2. Understand the subjunctive mood: The subjunctive mood in English is used to express demands, suggestions, or necessity. The form of the verb used in the subjunctive mood is always the base form (the bare infinitive without "to"), regardless of the subject (whether it is singular or plural, first, second, or third person).
3. Apply the rule to the verb: The subject of the "that"-clause is "every athlete" (third-person singular). Ordinarily, in the indicative mood, we would use the singular verb form "completes". However, because of the subjunctive trigger "It is essential that", we must use the base form of the verb, which is "complete".
4. Evaluate the options:
- "completes his registration before the deadline": Uses the indicative form "completes" instead of the subjunctive base form "complete".
- "completed their registration before the deadline": Uses the past tense, which is incorrect here.
- "should be completing their registration before the deadline": Unnecessarily wordy and changes the structure of the subjunctive clause.
- "complete their registration before the deadline": Correctly uses the base form "complete" to satisfy the subjunctive requirement.