Which correctly combines the clauses to avoid faulty coordination? More than 1,400 ME 262 turbojets were produced. Fewer than 300 saw combat.

'but'

This coordinating conjunction is the correct addition to the sentence in order to combine the two clauses. The first part of the sentence shows a huge number produces, but the second part shows that only some of them were used. The sentence should read:

More than 1,400 ME 262 turbojets were produced, but fewer than 300 saw combat.