If a VPC is deleted, the resources in that VPC are deleted. Does this include key pairs?

No key pair is not deleted

Can a key pair created in one VPC be used in another VPC?

Key Pair is regional level, can be used anywhere within the region

Can a security group created in one VPC be used in another VPC?

no, SG is specific to one vpc

What is Multi-part Upload? When is it used?

uploading of large files by splitting into smaller chunks and then uploading without uploading the entire file. it can be when dealing with large data file's

What is the default outgoing(outbound) rules set for Security Groups by AWS?

By default all outgoing rules are enabled in the security groups in AWS.