Firewall tiers?

I’m doing this practice test and it’s asking what tier of firewall it is. I can’t post the pic here but it goes like:

Internet | I Firewall —————- web server | | Router —————- VPN concentrator | | Switch | | ——————————————— | | | | | ( computers )

They are saying it’s a layered security network and asking which tier of firewall it is. My guess is single tier but I may be wrong. They have a choice of single tier to four tier.

Any help would be appreciated. Thanks in advance