The minimum distance between the centers of bulb of diameter du, of a multi under reamed piles, is A. 1.35 du B. 1.25 du C. 1.5 du D. 1.75 du

The correct answer is A. 1.35 du.

The minimum distance between the centers of bulb of diameter du, of a multi under reamed piles, is 1.35 du. This is to ensure that the piles are not too close together and that they have enough space to develop their full bearing capacity.

Option B, 1.25 du, is too close and would not allow the piles to develop their full bearing capacity. Option C, 1.5 du, is a safe distance but may not be the most efficient use of space. Option D, 1.75 du, is too far apart and would require more material and construction time.

Therefore, the minimum distance between the centers of bulb of diameter du, of a multi under reamed piles, is 1.35 du.