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

You would need a huge number of tanks to pin down an entire enemy trench line, and a long line of tanks is vulnerable to artillery and mortar fire.

Aircraft can only strafe a trench for a few minutes before using up their ammunition, and pilots hate such a dangerous job.

Poison gas will inconvenience the enemy but their gas masks will protect them. Then everyone on both sides will have to put on gas masks - cutting visibility and creating more confusion.

A well-executed "creeping barrage" will force the defenders to stay underground or deep in their trenches as your men advance.