Not necessary, because what matters is whether, if iron-60 were present early in the solar system’s history, it would be found in meteorites from that period. Whether the meteorite contains other elements that aren’t produced by a supernova has no bearing on whether it would have iron-60, which we’re told is produced by a supernova.

Not necessary, because the argument concerns our own solar system. How other solar system are formed has no bearing on what’s involved in our own solar system’s formation.

Not necessary, because even if supernovas DO produce a lot of other kinds of iron, the argument is based on our inability to find iron-60 in meteorites. There’s no indication that we’ve discovered other kinds of iron in those meteorites, so whether supernovas produced those other kinds doesn’t relate to the significance of these meteorites.

Not necessary, because the argument is based on our inability to find iron-60 in meteorites from early solar system history. Whether we’ve found iron-60 in later meteorites has no bearing on early solar system history.

Necessary, because if this were not true — if we might NOT find iron-60 in early solar system meteorites even if iron-60 actually was present in early solar system history — then the fact we haven’t found iron-60 in the meteorites would not establish that iron-60 was not present in early solar system history. (E) must be assumed in order for our failure to find iron-60 in the meteorites to support the conclusion.