a) 2.0 x nominal diameter of fastener
b) 3.0 x nominal diameter of fastener
c) 1.5 x nominal diameter of fastener
d) 2.5 x nominal diameter of fastener

Answer: d
Explanation: Pitch is centre to centre spacing of bolts in a row, measured along direction of load. Distance between centre to centre of fasteners shall not be more than 2.5 times nominal diameter of fasteners.