Your organization has a VPC set up with a custom route table having 40 routes for different use cases such as "VPC peering", "VPN connections", "NAT gateways" etc with different IP ranges. The Main route table had a local route to the internet gateway to act for the public subnet. The CIDR IP range for the VPC is 10.10.0.0/16, which will lead to multiple subnets. These subnets require a custom route table to be associated with them for the respective application(s). Many times, the teams forget to associate the custom route table to the subnets explicitly which leads to a lot of troubleshooting hours while connecting to the new subnets from the VPN. As an architect, how would you resolve this issue?

Answer options:

A.Create a script to create a new subnet and associate it with the custom route table. Share this with all the teams.
B.Make the custom route table as the main route table. Any new subnets created will get associated with it implicitly.
C.Delete the Internet Gateway route from the main route table.
D.Replace all routes from the custom table with the main route table and vice versa.

Answer: B
A custom route table can be made as to the main route table so that all implicit associations of subnets will now point to the newly set main route table. All the future implicit associations of newly created subnets will point to the newly set main route table.
Option A is incorrect, although subnet creation and association can be done programmatically, it may not be feasible to share access keys with all the teams (assuming the creation process is done on a remote network where roles cannot be used). It is also difficult for the organization to set up the process to run this script for new teams as they might not be aware of it.
As described above, option B is setting a custom table as the main route table is a simple configuration. All the associations would point to the new main route table implicitly.
Option C is incorrect because deleting the internet gateway does not solve the problem. It might create a new problem for EC2 instances using the NAT gateway to cause failures in connecting to the internet.
Option D is incorrect because it is tedious and error-prone.
With all the given options, option B is the best suitable solution.
Reference: 
https://docs.aws.amazon.com/vpc/latest/userguide/VPC_Route_Tables.html#CustomRouteTables