"Booking" is a verbal in this sentence. A verbal is a verb used as another part of speech. This particular verbal is a gerund. A gerund is a verb form ending in -ing that is used as a noun. Therefore, it can function in all the ways a noun functions: subject, object, appositive, direct address, predicate nominative, and objective complement. As a noun, it can also have adjectival modifiers. However, it still retains its ability to function as a verb and as such, can do those things a verb does:have a subject, adverbial modifiers, tense, objects, and be active or passive.
In this instance, "booking" is an appositive. An appositive is an noun set beside another noun to explain or identify it. It's in apposition to "question." It is also the object of the preposition "of." "tours" is the direct object of "booking."
I'm sure this is confusing to you. If I could diagram on the computer, I would show you how and why it is what it is. I hope the knowledge that it is a verbal is at least helpful to you.