most markets are not monopolies in the real world because

a. supply curves slope upward.
b. firms usually face downward-sloping demand curves.
c. firms usually equate price with marginal cost.
d. there are reasonable substitutes for most goods.

D. there are reasonable substitutes for most goods.

Explanation:

A monopoly is when there is only one firm operating in the industry. There are also no subsituites for goods and services produced by the monopoly. The monopoly sets the price for his product and earns economic profit in the long and short run.

There aren't a lot of monopolies in the real world because most goods have substitutes. Therefore, consumers can substitute the monopoly product for another product and there isn't just one firm operating in the industry.

d) social security number

let me know if i am correct or not, so i can edit the answer.