Your company is hosting a web application on AWS (with an autoscaling group already in place). According to the architectural best practices, the application must be highly available, scalable, cost-effective, with high-performance, and require minimal human intervention. You have deployed the web servers and database servers in the public and private subnet of the VPC, respectively. While testing the application via a web browser, you noticed that the application is not accessible.

Answer options:

A.Configure a NAT instance in your VPC and create a default route via the NAT instance and associate it with all subnets. Configure a DNS A record that points to the NAT instance public IP address.
B.Configure a CloudFront distribution and configure the origin to point to the private IP addresses of your Web servers. Configure a Route53 CNAME record to your CloudFront distribution.
C.Place all your web servers behind ELB. Configure a Route53 ALIAS-Record to point to the ELB DNS name.
D.Assign EIP`s to all web servers. Configure a Route53 A-Record set with all EIPs with health checks and DNS failover.

Answers – C
Option A is incorrect because (a) NAT instance is ideally used to route traffic from a private subnet to the internet via a public subnet, (b) NAT instance is not managed by AWS and requires to be configured and maintained by the user; hence, adding to the overhead, and (c) if not scaled, can cause a performance bottleneck. NAT Gateway is a preferred option over NAT instances.
Option B recommends using AWS CloudFront and configure the distributions Origin to the web server and then use an AWS Route 53 ‘CNAME’ for the CloudFront Distribution. CloudFront is highly available and accessible to the Internet. It would work better if the Origin for the AWS CloudFront Distribution were pointed to an AWS ELB rather than the Web Server itself. CloudFront doesn´t support IPs as origins.
Since the Origin would only be a Web Server, if this server goes offline for a period of time, the website would become unavailable. The content is not cached at the Edge location or if the TTL for the content expires. 
So, Option B is incorrect as well.
Option C is CORRECT. Because, (a) if the web servers are behind an ELB, the load on the web servers will be uniformly distributed. Hence, if any of the web servers goes offline or becomes non-responsive, the traffic would be routed to other online web servers; making the application highly available, and (b) You can use Route53 to set the ALIAS record that points to the ELB endpoint.
Option D is incorrect. Because the Auto Scaling group may terminate and relaunch a new instance. It is not suitable to configure the recordset that points to EIPs.