Your client says, "With the unrealized gains in my portfolio, I have almost saved enough money for my daughter to go to college in $8$ years, but educational costs keep going up$."$ On the basis of this statement alone, which one of the following appears to be least important to your client's investment policy?

a$.$ Time horizon.

b$.$ Purchasing power risk.

c$.$ Liquidity.

d$.$ Taxes.

$($I know the answer is liquidity but I do not know why, please explain$)$