Which of the following issues least impacts the implementation negatively of a Mean Variance framework to create a in the real world?

Government bond or Treasury Bill rates provide a reasonable estimate of future risk free rates for a desired investment horizon.

Some investments do not trade on a daily basis and should be dropped when constructing an asset allocation.

Restricted investments, investments that are not accessible by investors from a certain market, should not be considered when constructing an asset allocation.

The historical return for each asset is easy to calculate but estimating future returns are difficult to forecast

The historical correlation between each asset in the portfolio is easy to calculate but estimating future correlation each asset and another asset in the portfolio is difficult to forecast