The correct answer is A. Since
Explanation:
In writing, transitions are words or even phrases that connect different ideas in texts and establish relationships between them. Because of this, transitions are of many types depending on the type of relationship they establish, this includes transitions of addition, sequence, contrast, emphasis, consequence or effect, etc. In the case of transitions of cause and effect which are the type of transitions the sentence presented requires, this includes the words because, as, due to, since that express cause and the words so, therefore, thus, as a consequence that describe effects.
This implies in the case of the sentence "______NASA needed to determine the cause of the fire, they compared the burned-out remains of Apollo 204 with Command Module 014, a sister ship" from the options provided the one that is "Since" because this is one of the transitions that expresse causes and therefore it is correct to use it to explain the steps Nasa took because they neede to find the cause of the fire.