Which of the following is good practice when contacting another station in a contest?
Why Contests are about maximizing the number of valid contacts per hour, so the standard practice is to keep each exchange as short as possible: the other station's call if needed, your call, and the required exchange (typically a signal report plus a serial number, state, section or grid square). Anything beyond identification and the exchange just slows the rate and clogs the frequency. FCC identification rules still apply, so you always send your full assigned call sign, just nothing extra.
Watch out Signing only the last two letters is a tempting shortcut used in some pileups, but it is not proper identification under Part 97 and the pool treats it as poor practice; duplicating a contact wastes time and usually scores zero anyway.
Contest style: full call, exchange, done. No chit-chat, no partial calls, no dupes.