Answer: C

The diagonals of a rhombus are perpendicular and bisect each other, so they divide the rhombus into $4$ congruent right triangles. Each triangle has a hypotenuse of $13$ and a leg of $\frac{10}{2} = 5$, so its other leg is $\sqrt{13^2 - 5^2} = \sqrt{144} = 12$. The other diagonal is $2 \times 12 = 24$ inches, and the area is $\frac{10 \times 24}{2} = 120$ square inches.

Choice D multiplies the side length by the diagonal, $13 \times 10$, as if the rhombus were a rectangle.

Why the other choices are wrong

• A. 60 is the area of 2 of the 4 right triangles, half the rhombus.
• B. This takes half of 13 × 10, but the area of a rhombus is half the product of its diagonals.
• D. This multiplies the side length by a diagonal, as if the rhombus were a rectangle.