An Amazon VPC public subnet is defined by having a route to an Internet Gateway; a private subnet has no such route and uses a NAT Gateway in a public subnet for outbound-only internet access. This classic two-tier pattern gives the web tier inbound reachability while keeping the app tier inbound-isolated but still able to download patches.

Plain-Language Explanation:

Think of the VPC as a shopping mall. The food court (public subnet) opens to the parking lot through the front door (Internet Gateway). The staff stockroom (private subnet) has no public door, but a one-way service lift (NAT Gateway) lets staff order supplies from outside without letting customers walk in.

Option Analysis:685

B fails because Security Groups cannot make a public subnet act like a private one; route-table design is the control. C reverses the roles and breaks user access. D uses a VGW which is for VPN to on-premises, not internet egress.