A long shaft with a diameter 'd' is subjected to a twisting moment at its ends. What is the maximum normal stress acting at its cross-section?
a) τₘₐₓ = T / (π d³ / 16)
b) τₘₐₓ = T / (π d⁴ / 32)
c) τₘₐₓ = T / (π d² / 16)
d) τₘₐₓ = T / (π d³ / 32)