Given the time frame involved (~37 years) you have time on your side. Personally I would go for a highish risk one, but it all depends on your own personal aims. Do you want a pension that will be safe and provide a reasonable return or are you happy to take the risk of a loss in the hope of gaining a higher return.
It's a question that nobody can answer for you.
In the past, given a timeframe such as yours, a riskier option would have provided far higher returns. That doesn't guarantee that the same will happen for you, but the risks of it turning out very poorly over a term of +30 years aren't too high. If you go for a pension that is really "safe", you may end up with returns that don't keep the sum in line with inflation, so no such thing as a true no risk.
Personally, at 28, I'd go for the highest risk option and change the make up of the pension as the term reduces.