ANS:B - Triangular pitch

If the fluids in a shell and tube heat exchanger are clean and non-fouling, the best tube arrangement would typically be the triangular pitch. In the triangular pitch arrangement, the tubes are arranged in a triangular pattern. This arrangement provides better heat transfer performance compared to the square pitch arrangement because it promotes more efficient fluid flow and reduces the likelihood of fouling or buildup on the tube surfaces. Additionally, the triangular pitch arrangement allows for easier cleaning and maintenance of the heat exchanger. Therefore, the correct option is: Triangular pitch.