Which trait would best indicate that a particular arthropod was a member of subphylum chelicerata?

Two body segments.

Explanation:

The trait that would best indicate that a particular arthropod was a member of subphylum chelicerata is that it possess two body segments instead of three; A fused head and thorax (cephalothorax), and an abdomen (opisthosoma). Their segmented body with jointed limbs is covered in a cuticle made of chitin and proteins.

Genetic engineering is the answer.