NASA __ does many tests before flying a new spaceship, __ the Boeing Starliner didn’t reach the space station. It went to the wrong orbit. Now the ship is __ in space.
The answers are: c, a, b; look:
NASA _always_ does many tests before flying a new spaceship, _yet_ the Boeing Starliner didn’t reach the space station. It went to the wrong orbit. Now the ship is _still_ in space.
The first answer. Always refers to repeated, cyclic and unchanging conditions, habits, protocols or procedures. ‘Still’ would be grammatically okay here, but it is needed elsewhere in the passage, and in that place where it belongs, the other choices would also work grammatically but not be the best and/or not make sense.
The next answer, ‘yet’, provides a contrast with the information and adverb ‘always’ in the first answer; ‘always’ indicates something continuous or repeating without fail under certain conditions–and thereafter, ‘yet’ tells us a break in that consistency is coming. ‘Yet‘ also acts like the conjunction ‘but’.
Finally, the last answer, ‘still’ tells of a thing that has not ended. Again, all the options work in the space, but then in the other combinations would not make sense.