The correct answer is: A. is going to
The verb “to go” is used to express a future action. In this case, the sentence is about Olga’s future action of going to the gym. The present progressive tense (is going to) is used to express a future action that is planned or expected.
The other options are incorrect because they do not express a future action. Option B, “is going,” is the simple present tense, which is used to express a habitual or repeated action. Option C, “will goes,” is the simple future tense, which is used to express a future action that is not planned or expected. Option D, “will go,” is also the simple future tense, but it is the correct answer if the sentence is about Olga’s intention to go to the gym.