A Corner Point Solution Is Always the Optimum for a Consumer

A corner point solution is always the optimum for a consumer when

A) a unique point of tangency exists between the consumer's indifference curve and the budget line
B) the consumer has straight line (constant slope) indifference curves
C) there is no unique point of tangency between the consumer's indifference curves and the budget line and the consumer does not have straight line indifference curves.
D) the consumer is indifferent to both goods equally.