Concept:A prepositional phrase must correctly express the time relationship in the sentence.
Explanation:The blank needs a phrase showing when the aviation advances took place.
“During” means throughout a particular period, so it correctly matches the past tense “were”.
“Until” suggests the advances stopped at that time, which changes the meaning.
“By” means no later than a deadline and does not fit naturally here.
“Since” is used with present perfect or continuous tenses, not with simple past “were”.
Therefore, “during the Second World War” is the correct phrase.
Answer:during the Second World War.