I agree with Oscar A. that the correct answer your question is "C) gets." As Oscar mentions, gets is the third person singular present tense form of the verb get, and indeed a simple tense form of the main verb is required in a time clause that begins with the three-word subordinating conjunction of time as soon as. And unlike in many other languages, English requires the present tense in most clauses of time even if the time reference is in the future; cf. when I arrive tomorrow vs. French quand j'arriverai demain, literally 'when I will arrive tomorrow'. This is why, then, neither answer choice "B) is going to get" nor "D) will get" are correct because both of these show future-time forms of get.
Also, the meaning of as soon as requires reference to a specific point of time, usually the point at which another action takes place. This is why answer choice "A) is getting" is not correct since because the verb form shows the continuous or progressive aspect (be + -ing), that is, pointing to an action ("getting") or state that is not finished or complete at the time. In other words, in the "formula" X as soon as Y, "Y" must be complete or at least have started before "X" can be true. And in reference to your specific example, all that matters is that point at which Mark "gets there," i.e., arrives and whose state then changes from "not being there" to "being there"; then, and not before, is when you must "let me know."