Answer:
The correct answer is "The explorers discovered gold in California".
Step-by-step explanation:
The other sentences are in different tense:
1. "The explorers were discovering gold in California" is in past continuous tense.
2. The sentence "the explorers are discovering gold in California" is in present continuous tense.
3. And the sentence, "the explorers will discover gold in California" is in future tense.