Explanation
Correct answer: D.
Choice D is the best answer. The convention being tested is the coordination of main clauses. This choice correctly uses a comma and the coordinating conjunction “but” to join the first main clause (“Materials…Ru”) and the second main clause (“the alloy…NiCoCr”).
Why the other choices are wrong
Choice A
Choice A is incorrect because it results in a run-on sentence. The two main clauses are fused without punctuation and/or a conjunction.
Choice B
Choice B is incorrect because when coordinating two longer main clauses such as these, it’s conventional to use a comma before the coordinating conjunction.
Choice C
Choice C is incorrect because it results in a comma splice. Without a conjunction following it, a comma can’t be used in this way to join two main clauses.