Considering that complementary goods have a negative cross of elasticity, it is correct to assume that the increase of the price or demand of one goods will result in an increase of price and demand of the other.
By that only, we can rule out answers A and B.
Since the products, as the name states, are complementary, there is no logic associated with the idea of consuming more of one good meaning less of the other (considering what was stated above), so it can't be the letter C.
It is D
"An increase in the demand for one will usually result in an increased demand for the other"
