The surface area of a cone can be found using the formula: surface area = π * r * (r + l), where r is the radius and l is the slant height.
In this case, the radius is given as 10 inches and the slant height is given as 15 inches.
Substituting these values into the formula, we have surface area = 3.14 * 10 * (10 + 15).
This simplifies to surface area = 3.14 * 10 * 25 = 785 square inches.
Therefore, the correct answer is 785 square inches.