The correct answer is D. spoke.
Gandhi Ji was a great leader who always spoke the truth. He believed that truth was the most important thing in life, and he always tried to live his life according to this belief. He was also a great orator, and he used his speeches to inspire and motivate people.
The other options are incorrect because they do not agree with the tense of the sentence. The sentence is in the past tense, so the correct answer must also be in the past tense. Option A, speak, is in the present tense. Option B, speaks, is in the present tense. Option C, speaking, is in the present participle tense.
I hope this explanation is helpful. Please let me know if you have any other questions.