Let's analyze each option to understand why "is going" is the correct choice in this context:
The sentence is: "Olga ________ to the gym this evening."
(A) will go - This is the future simple tense. While it's grammatically correct, the use of "this evening" suggests a more immediate future, making the present continuous tense a more natural choice.
(B) will goes - This is grammatically incorrect. When using "will," we use the base form of the verb, so it should be "will go" instead of "will goes."
(C) is going - This is the present continuous tense. It indicates an action that is happening at or around the present moment. In this context, it is suitable because Olga's going to the gym is something planned for this evening.
(D) is going to - This is also correct. It is the present continuous tense with the preposition "to." It is commonly used to express future plans or intentions. So, "Olga is going to the gym this evening" is a valid and natural construction.
Both (C) "is going" and (D) "is going to" are acceptable, but (D) adds a bit more emphasis on the planned nature of the action. Therefore, both (C) and (D) are correct, but (C) is slightly more concise. The complete sentence is: "Olga is going to the gym this evening."