The best word to replace the infinitive in the sentence is "to assemble."
Explanation:
1. In the original sentence, the infinitive "to assemble" is used after the verb "completed" to indicate the purpose or goal of the scientist's action.
2. The word "assembling" is a gerund form that functions as a noun and would not be suitable in this context as it does not convey the intended meaning of an action completed by the scientist.
3. "Assemble" is the base form of the verb and would not be appropriate here as it does not match the structure of the sentence where the scientist has already completed the action.
4. "Assembled" is the past tense form of the verb and would disrupt the flow of the sentence since it would change the tense from past perfect to simple past, altering the intended meaning.
Therefore, "to assemble" is the best word to replace the infinitive in the sentence to maintain the correct tense and convey the scientist's action after completing the assembly of the components.
