Which group of arthropods is the most diverse and has members with complete metamorphosis?

Indeed it is ! I take it you asked not knowing and then researched the answer after posting the question...

Joey100 - "Nothing even comes close to challenging the Hexapods on Diversity"...since you have not stipulated at a certain rank and have indeed compared 2 different rank levels by way of comparison, the bacteria are estimated to be more diverse than hexapoda, perhaps ten fold or more.

EDIT 2
In fairness, it was more than reasonable to assume you meant phylum-ish level (I did). Nonetheless I was just seeking clarity.

Are you still asking? Just to correct your answer?

It is most definitely A.

The Hexapoda are essentially the insects, with a couple of other little groups added on who used to be considered insects but aren't anymore. With well over a Million species of Insect (contributing about 3/4 of the Animal Kingdom) the insects (or Hexapods in this case) are most definitely the group showing the greatest diversity.

The Hexapods are sub-phylum, and no other sub-phylum in Life comes close to challenging the Hexapods on Diversity. Even known Phyla struggle to challenge the Arthropods on Diversity
 The closest anything gets is the Phylum of Angiosperms in the Plant Kingdom, with just over 250,000 species. Even this does not challenge the Hexapods.

Edit: I see, I didn't make myself clear enough. I was thinking of Phyla-level organisation. I know Hexapods aren't a phylum, they're a sub-phylum, but I was still thinking of that as a phylum-like division of organisation. They're certainly not a Class. I see I didn't make that clear enough.