Which of the following sentences is correct?

In option A, since the word 'though' is used, a contrast has already been drawn; therefore, the use of the word 'but' is redundant. Option A is not grammatically sound. Option B uses a double negative ('not inactive') and the phrasing sounds unnatural. Option C presents a sentence that fails to draw the right contrast (making the use of the word 'but' sound misplaced), further, 'my grandfather is without activities' is not sensibly phrased. Option D is correct as it presents the proper usage of words and has the correct phrasing.