a) 0.7d, where d=nominal rivet diameter
b) 0.5d, where d=nominal rivet diameter
c) 0.43d, where d=nominal rivet diameter
d) 0.25d, where d=nominal rivet diameter

Answer: c
Explanation: Diameter of head for
(i) button head rivet = 0.7d,
(ii) flat countersunk head rivet = 0.43d,
(iii) flat head rivet = 0.25d, where d=nominal rivet diameter.