Statistics and Probability

suppose that potential customers were offered a free stay at the hotel. After they accepted the offer, they were randomly allocated to one of two treatments: a free one or free two-night stay. Which of the following does NOT pose a threat to the interpretation of estimated effect of Stay as casual?

a. Not everyone who was offered the free stay accepted the offer

b. Not everyone in the experiment completed the customer satisfaction survey.

c. If the guests stay at the same time, they may discover the existence of the second treatment

d. Socio-demographic characteristics of the customer (such as their income) have not been included as covariates in the regression

Please send me answer ASAP.