Answer:
When much oil was discovered in the land of the Nigerian Igbos in 1965, they wanted to rule themselves, and a bloody coup began, starting a war over Biafra, their new country.
Step-by-step explanation:
The most effective combination of the given sentences is the first one. It's easy to understand and grammatically correct.
The second one is incorrect because the sentences aren't well-connected. We shouldn't use the same conjunction too many times, and here, the conjunction and is repeated.
In the third option, the same issue is present. Besides that, an unnecessary that is used before the phrase much oil, making the sentence additionally difficult to comprehend.
In the fourth option, we have a comma splice - a type of punctuation error, where independent clauses are connected only by a comma. Inserting a conjunction or replacing the comma with a semicolon would correct this mistake.
This is why the first option is the correct one.