Which fin has the most efficient design?

a$)L=10cm,m=0\mathrm{.}2c{m}^{-1}$

b$)L=12cm,m=0\mathrm{.}1c{m}^{-1}$

c$)L=8cm,m=0\mathrm{.}14c{m}^{-1}$

d$)L=6cm,m=0\mathrm{.}25c{m}^{-1}$