Answer:
"but"
Step-by-step explanation:
The best and only transition that works for this sentence would be "but". This states that even though the ride was safe the engineers still decided to go against the logical next step and maintain the ride closed until they could make certain that it was completely safe. If the second part of the sentence was a logical conclusion the transition could be "so" or "and". But that is not the case so this would be the best transition for the sentence.