Similar question(s) asked/answered
here.
While I no longer invest with Fidelity (consolidated accounts at Vanguard) investment choices are pretty much generic.
The primary question you have to answer is how long do you have until retirement? While the answer if a function of what other investments you may or may not have, generally speaking, your investment horizon sets your asset allocation: the longer you will leave the money invested, the higher the percentage in stocks should be. In the long run, stocks will outperform other assets so unless you are going to need (withdraw/spend) your 401(k) funds "soon" (1-5 years or so) you should put it (all) in stocks.
Simplest answer: 100% S&P500 and forget about it.
Want to assume a little more risk for a chance at greater return? Put a small percentage (5-15%) in their
developed international fund.
Willing to accept even more risk for a chance of even greater return? Put a little (5% or so) in their emerging market fund.
Note that with the international choices you will incur exchange rate (currency) risk but, like so many things, in the long run it's a push.
If you think you can time the economy to a degree (i.e. it's a safe bet we are not in a growth cycle right now, at least not a robust one) shift some from the S&P to growth and/or small caps when the economy starts to (really) improve (i.e. when a Republican takes the WH; J/K...sort of).
All the retirement date funds do is, as the retirement date approaches, shift more and more of your funds from stocks (S&P500 or equivalent) to bonds (government securities). It's not a bad concept but IMO they are generally too conservative, investing far too much in bonds.