WW1 experience shows that "C" is the best answer.

German WW1 defences by 1917 were so strong that even artillery shells could not destroy them.

Tanks lack the firepower to destroy them, and they can become targets themselves as they advance.

Underground mining would work - but you'd need a lot of tunnels and explosives and the resulting craters would become an obstacle to your own soldiers.

Pill boxes and bunkers can be flanked and taken from the rear with nerve and good timing.