Choice "B" is correct. The change in demand for a product, based upon a given change in that product's price, is dependent on whether or not other (presumably cheaper) goods can be substituted for the product.
Choice "a" is incorrect. The elasticity of supply (not demand) would take into account the response time producers might have to market changes.
Choice "c" is incorrect. The number of sellers is irrelevant when calculating the elasticity of demand.
Choice "d" is incorrect. A complement good's demand is the same as the primary good. For example, an increase in the demand for a given food would cause the demand for its complement to also increase. The increased demand of the complement is irrelevant when calculating the elasticity of demand.