The decision that would be considered part of the "product" element of the marketing mix is:
Whether to make a product blue or red
This decision pertains to the product's design and attributes, which are core components of the product element of the marketing mix. The other options relate to placement (shelf/table), pricing ($5 or $10), and promotion (sale on product A or B), which are part of other elements of the marketing mix (Place, Price, and Promotion, respectively).