Final answer:
The correct answer is option 'D) but, and', because 'but' provides a contrast between the need to cross the river and the fact the bridge is gone, while 'and' connects the resulting situation with the bridge's condition.
Step-by-step explanation:
To complete the sentence "We needed to cross the foaming river to get back to camp, _______ the river had washed the bridge away, _______ all that was left was a few pieces of broken wood," we need to choose the appropriate conjunctions that fit best in the blanks.
The sentence structure requires a conjunction that contrasts the first independent clause with the second clause and then a coordinating conjunction to connect the last part of the sentence. The correct options for the conjunctions are 'but' and 'and', respectively, making the complete sentence:
"We needed to cross the foaming river to get back to camp, but the river had washed the bridge away, and all that was left was a few pieces of broken wood."
Therefore, the correct answer is option 'D) but, and', which I will mention as the correct option in my final answer.