Market Penetration is:
The % substitutes have replaced competitors
The % of an individual customer's spending on your product(s) vs. competitors
The % of geography in a market where your product(s) are available
The % of dollars in a market spent buying your product(s)
The % of buyers in a market that are customers purchasing your product(s)
Answer: The % of buyers in a market that are customers purchasing your product(s)