The correct answer is B. Negative.
An indifference curve is a graph that shows all the combinations of goods that a consumer is indifferent between. The slope of an indifference curve is always negative, which means that as a consumer consumes more of one good, they must consume less of another good in order to remain indifferent. This is because consumers have diminishing marginal utility, which means that each additional unit of a good provides less satisfaction than the previous unit.
Option A is incorrect because the slope of an indifference curve is always negative. Option C is incorrect because the slope of an indifference curve is never vertical. Option D is incorrect because the slope of an indifference curve is never parallel.