Variable, based on the performance of the S&P500 Index. The only thing that is fixed in terms of the value is the guaranteed minimum return, which is 100% of the amount you invest if you choose the 10-year option, 140% of the amount you invest if you choose the 15-year option, and 160% of the amount you invest if you choose the 20-year option, as long as you stick to the plan in all cases.
The guaranteed minimum values are there as a safety-net for you just in case there is a huge and catastrophic stock market crash just before the end of your chosen duration. It is highly likely that your actual pension value, in future, will be significantly higher – the long-term average return of the S&P500 Index is just under 10% per year.