Select an option to complete the given sentence.

_______ had the researchers completed their preliminary trials, ________ fresh complications emerged that altered the entire direction of the study.

This sentence needs correlative conjunctions that show two events happening in immediate succession - the completion of trials and the emergence of complications.

Option 1 -> "No sooner...than" is the correct correlative pair used to indicate that the second event happened immediately after the first event.

Option 2 -> "Scarcely...before" would incorrectly suggest that complications emerged before the trials were completed, which reverses the logical sequence.

Option 3 -> "Hardly" must be paired with "when" or "before," not "that." This pairing doesn't exist in standard English.

Option 4 -> "No sooner" is always paired with "than," never with "that." This is an incorrect combination.

Hence, Option 1: No sooner, than -> forms the proper correlative conjunction pair that expresses immediate succession of events -> correct