What is the main difference between an asset index and a portfolio of assets:

a$.$ An index is risk$-$free, a portfolio bears risk.

b$.$ An index is a collection of assets that fall into the same category, a

portfolio can be any grouping of investments.

c$.$ An index can only be bought from a broker, whereas a portfolio can be

constructed by any individual investor.

d$.$ A portfolio is publicly traded on a large stock exchange like NASDAQ, an

index is only privately available