What is the purpose of DNS in a network?

DNS converts domain name into Ip address

How can you improve latency for an application hosted in a single AWS region?

you can use Amazon Cloud front

if entered aws --version cmd in my powershell but cmdlet is invalid. why and what i have to install?

AWS CLI is not installed , you should install it

how could you able to connect to an instance private subnet which dont have a public IP to the internet?

you can have have jump server or nat gateway

without NAT gateway is there any possiblity to connect a private ec2 without a public IP to access data in s3 bucket?|

you can use VPC gateway endpoint for S3