a. Incorrect. Remember that a decline in price causes movement down along a demand curve (thus increasing quantity demanded), but price does not cause a shift in demand.
b. Correct. An increase in the price of a consumer substitute will cause some consumers of tofu-burgers to shift to veggie-burgers, causing an increase in the demand for veggie-burgers.
c. Incorrect. Buns are a complement to burgers, and an increase in bun prices will reduce demand for burgers.
d. Incorrect. This will increase the supply curve, not the demand curve.