Which option of bonding multiple strands from Cat5 pick up the lowest level of EMI?
Your best bet would be to bond 1 wire from each twisted pair together; so to keep it simple, bond the 4 dashed wires together, and the 4 solid wires together.

From our good friends at Wikipedia:

"Twisted pair cabling is a type of wiring in which two conductors of a single circuit are twisted together for the purposes of canceling out electromagnetic interference (EMI) from external sources"

So, each solid/dashed pair should cancel each other out.