Answer:
change in buyer tastes.
Explanation:
Buyers now want more donuts than the other goods.
increase in the cost of donuts is more likely to decrease the amount of donuts consumed, as they would be more expensive
consumers expecting something doesn't necessarily change anything, as they haven't yet dropped and therefore it's unlikely they'd increase the demand until they actually were cheaper.
Coffee doesn't necessarily effect donut sales, unless stated in the question.