Step 1: List the genuine functional parts of the Ti plasmid: T-DNA (transferred DNA with oncogenes and opine synthesis genes), vir genes (encode the machinery for T-DNA processing and transfer), ori (origin of replication so the plasmid can be maintained in the bacterium), and tra/trb genes (mediate plasmid conjugative transfer between Agrobacterium cells).
Now compare with cos region. A cos site is a specific palindromic sequence recognized by terminase enzyme during lambda phage DNA packaging into the phage capsid, a concept from phage genetics and cosmid cloning vectors, unrelated to Agrobacterium Ti plasmid structure.
Since vir, ori and tra/trb are all real Ti plasmid regions and cos is borrowed from a completely different biological system, the odd one out is cos region.