Final answer:
The demand curve slopes downward and is concave to the origin due to these effects.
Step-by-step explanation:
A demand curve exhibits a concave shape due to the substitution and income effects of a price change. When the price of a good decreases, the substitution effect encourages consumers to switch from relatively more expensive goods to the now relatively cheaper good. This leads to a higher quantity demanded, showcasing the downward slope of the curve. Simultaneously, the income effect increases the consumer's purchasing power, allowing them to buy more of the good. However, as the quantity demanded increases, the diminishing marginal utility results in a less steep slope, creating a concave shape as it moves along the demand curve. This concavity signifies that individuals are willing to buy more of a good at lower prices but with diminishing willingness for additional units.
The correct answer is: d. It is concave to the origin.