Which of the following is an odd-order intermodulation product of frequencies F1 and F2?

Explication

The order of a product mF₁ ± nF₂ is |m| + |n|, so 2F₁−F₂ is order 2+1 = 3 and is odd-order. The other two listed products are even-order: 3F₁−F₂ is order 4 and 5F₁−3F₂ is order 8, so the claim that every listed product qualifies is false.