The correct answer is A. Convex.
An indifference curve is a graph showing combinations of goods that provide the same level of satisfaction to a consumer. Indifference curves are usually drawn as convex curves, meaning that they are bowed inward towards the origin. This is because consumers generally prefer to have more of both goods, rather than just one or the other.
If an indifference curve were concave, it would mean that consumers would be indifferent between having a lot of one good and a little of the other. This is not a realistic assumption, as consumers generally prefer to have more of both goods.
An L-shaped indifference curve would mean that consumers would be indifferent between having a lot of one good and none of the other. This is also not a realistic assumption, as consumers generally prefer to have at least some of both goods.
A vertical indifference curve would mean that consumers would be indifferent between having any amount of one good and no amount of the other. This is also not a realistic assumption, as consumers generally prefer to have at least some of both goods.
In conclusion, indifference curves are usually convex because consumers generally prefer to have more of both goods.