a) 1.60d, where d=nominal rivet diameter
b) 2.5d, where d=nominal rivet diameter
c) 3d, where d=nominal rivet diameter
d) 5d, where d=nominal rivet diameter

Answer: a
Explanation: Diameter of head for
(i) button head rivet = 1.6d,
(ii) flat countersunk head = 1.5d,
(iii) flat head = 2d, where d=nominal rivet diameter.