The correct answer to this question is a decrease in the price of the flour because white bread uses flour as one of its primary ingredients, the price of this commodity will affect the cost to produce white bread. If the raw materials used in bread decrease in price, the cost of making the bread also decreases.
Economic theory then says that as price decreases demand and quantity will increase, as people are able to buy more of this bread to feed their families.
Although the prices of substitutes (like rye bread) and complements (like butter) may affect the overall demand for white bread, a decrease in the price of flour has the most direct influence on the equilibrium price and quantity for this demand.