Which of the following solutions will provide the required protection?

Explanations:

Using an S3 VPC endpoint restricts S3 access to resources within a specified VPC, ensuring that only EC2 instances in that VPC can access the S3 bucket. Coupled with an S3 bucket policy that explicitly allows access only from the VPC endpoint, this solution effectively mitigates the risk of a malicious insider in a different VPC gaining access to the documents.

While using EC2 instance profiles can limit access to the S3 bucket based on IAM roles, it does not restrict the VPC access. A malicious insider could still create an EC2 instance in another VPC that has the same role and gain access to the S3 bucket, hence this does not provide the necessary protection against unauthorized access from other VPCs.

Client-side encryption would protect the documents themselves but does not prevent access to the S3 bucket. If the instance is compromised, an attacker could potentially access the encrypted documents and the decryption key from the instance metadata, leading to a security breach.

Server-side encryption provides protection for data at rest but does not limit access based on the VPC. Without restricting access to the S3 bucket through VPC endpoints or specific IAM policies based on VPC, a malicious insider could still access the bucket from a different VPC and obtain the encrypted data.