Who faces centers on the line of scrimmage?

Answer 1

It all depends on what defense you are running, and what play is being called by the Defense.

In the 3-4 defense (or any formation starting with "3" - as the first number is the number of defensive lineman on the line of scrimmage), the Nose Guard/Tackle would line up right over the ball and his first job would be to take on the center.

In a 4-3 - (with 4 defensive linemen on the line of scrimmage) - you aren't necessarily playing the center man to man. Unless you are running a "stunt" - the 4-3 usually carries the notion that the 2 inside D Linemen (on either side of the Center) are going to be taking on the two Guards on either side of the Center - engaging them or passing by them with a "move".

A Nose Guard/Nose Tackle's job most times is to occupy the Center and then willingly absorb one or two of the Guards at the same time, as to leave no O Linemen to pick up his Linebackers - leaving a possible free-ride to the QB, or filling a "gap" that the ball would be run through by one of the Backs.

The above is "typical", but NOT the only options for defensive formations.

It really depends on the play call.