Laelle Airlines, an international airlines company, offers a variety of services to its customers. Given this information, which of the following is the core service provided by Laelle Airlines?

a. Providing plush seats with comfortable legroom for its passengers

b. Offering souvenirs to all the passengers after their travel

c. Offering three-course meals for the passengers who travel in the airlines

d. Providing personalized toiletries, including wet towels and sleep masks, for all passengers