Investors nearing retirement will typically shift their investment portfolios to include risk investments.

I believe the answer is: Risk

Portofolios refers to the diversification of investment. Meaning that you are not putting all your capital into one type of investment. This would reduce the risk of investment because you won't lose all of your capital when one of your investments fail.

to answer your question:

f(x)= a^x.  if x=0, f(x)=1  if x=1, f(x)=a  if x=1, f(x)=a^a