Questions: Market share is the portion of total sales in a product category accounted for by a particular brand.

The answer is: share.

Explanation for each option:

1. Rival: This term refers to a competitor in the same market or industry. It does not describe the portion of total sales accounted for by a particular brand.

2. Share: This is the correct term. Market share specifically refers to the portion of total sales in a product category that is accounted for by a particular brand. It is a common metric used to gauge the competitiveness and success of a brand within its market.

3. Advantage: This term generally refers to a condition or circumstance that puts a company in a favorable or superior position. While having a large market share can be an advantage, the term "advantage" itself does not describe the portion of total sales.

4. Mix: This term often refers to the variety of products or services that a company offers. It does not describe the portion of total sales in a product category accounted for by a particular brand.

Therefore, the correct answer is "share."