If talking about SSL, the original single EC2 instance setup should have an SSL certificate deployed on the web server. Hence, I don’t think NLB not supporting SSL termination is a concern here. I agree session stickiness can be the reason for the ALB
This question is about elimination and you are correct in that sticky sessions is the reason why an ALB is the best answer here…because NLB does not support that. On the exam, they will try to get you to follow false leads like the SSL situation.