The correct answer is: B. negative.
Complementary products are goods that are used together. For example, pens and paper are complementary products, because you cannot use a pen without paper. If the price of pens goes up, people will buy less pens, and therefore they will also buy less paper. This means that the cross-price elasticity of demand for paper with respect to the price of pens is negative.
A positive cross-price elasticity of demand would mean that the two goods are substitutes. For example, coffee and tea are substitutes, because if the price of coffee goes up, people will buy more tea.
A zero cross-price elasticity of demand would mean that the two goods are unrelated. For example, the price of pens has no effect on the demand for paperclips.