Final answer:
Market share is an external backwards-looking metric because it reflects a company's past performance and its position in relation to external competitors within the industry.
Step-by-step explanation:
When considering marketing metrics for a company, market share is most likely to be D) an external backwards-looking metric. Market share represents the percentage of an industry's sales that a particular company controls during a specific time period compared to its competitors. As a metric, it looks at past performance in the market and is externally focused because it compares a company's sales to that of the larger market, which includes the activities of other competing businesses. It's used to assess a company's competitiveness and overall command of the market relative to other players.