Pick the thing closest to a low-expense total market (or S&P 500) index fund. It should have the word "index" in the name and be much cheaper than most of the other stock-type funds.
If your 401K doesn't have one, complain to the committee that oversees the plan because (IMO, at least) any plan that fails to offer a basic cheap stock index fund is failing to meet its fiduciary responsibility.
Note that you should not exactly "invest in the lowest fee fund you can" as Zaga recommends because that's often a money-market fund (i.e., almost zero return). Filtering out everything that isn't a stock fund and then doing that is probably okay, though.