Choose the option that completes the sentence correctly:

I ______ hard until I crack the MBA entrance examinations.

In the given sentence "I will study hard until I crack the MBA entrance examinations," the most appropriate option to complete the sentence correctly is "will study."

The sentence is in the future tense, and the phrase "until I crack the MBA entrance examinations" suggests a continuous effort in the future. The use of "will study" indicates the speaker's intention to study hard in the future with the goal of cracking the MBA entrance examinations.

The other options do not convey the correct sense for the given sentence. Option B, "Am studying," is in the present continuous tense, which does not fit well with the future-oriented meaning of the sentence. Option C, "Was studying," is in the past continuous tense, which is not appropriate for expressing a future plan. Option D, "Study," is in the simple present tense, but it does not express the continuous effort and determination that the sentence implies.

Therefore, the correct answer is "A. Will study."