Not necessary, because the difficulty of equipping engines with short and long nozzles is irrelevant. What matters is whether a rocket NEEDS short and long nozzles on their engines.

Necessary, because if it were not true — if some rockets do NOT pass through the thin upper atmosphere during their ascents — then we cannot conclude that “all” rockets need to have long nozzles. For the rockets that don’t pass through the thin upper atmospheres, we have no reason to think their engines need to have long nozzles. So the author must assume (B) in order to conclude that “all” rockets need long nozzles.

Not necessary, because even if rockets with only short nozzles on the engines CAN reach high altitudes, what matters is whether the rockets can work MOST EFFECTIVELY with only short nozzles. And we know that in the thin upper atmosphere, long nozzles are more effective than short.

Not necessary, because the argument is concerned with what allows rocket engines to be MOST effective. The author doesn’t have to assume that anything is required to be “effective”; only what is required to be MOST effective.

Not necessary, becauase the conclusion asserts only that to work most effectively, rockets must have short and long nozzles “on their engines.” This doesn’t require that a single engine needs to have both short and long nozzles. A rocket could have multiple engines, some with short nozzles and others with long nozzles. That would still be a rocket that has short and long nozzles on its engines. Also, although we know in the thin upper atmosphere there is a LOWER atmospheric pressure than at low altitudes, it’s not clear that this constitutes a “low” atmospheric pressure.