Which one of the following protocols is NOT a type of VPN $($Virtual Private Network$)$ when configuring a VPN connection in Windows?a$.$SMTP $($Simple Mail Transfer Protocol$)$b$.$PPTP $($Point$-$to$-$Point Tunneling Protocol$)$c$.$L$2$TP $($Layer $2$ Tunneling Protocol$)$d$.$SSTP $($Secure Socket Tunneling Protocol$)$