The voice$-$over$-$IP CODEC standard with the least bandwidth and is best suitable for a very busy network, such as in a Wide Area Network is:

a$.$G$\mathrm{.}726$

b$.$G$\mathrm{.}729$

c$.$G$\mathrm{.}723$

d$.$G$\mathrm{.}711$

e$.$G$\mathrm{.}728$