Final answer:
The opinion that all people can overcome their fear of needles is unsubstantiated as it is not supported by facts within the provided information and does not consider individual psychological differences.
Step-by-step explanation:
The opinion from the selection that is unsubstantiated is D) All people can overcome their fear of needles. While opinions A) It can be quite convenient to donate blood, B) Donating blood is very safe, and C) The blood supply is constantly in jeopardy, are supported by facts and data, the claim that all people can overcome their fear of needles is not substantiated within the provided information. This opinion does not consider individual psychological differences and the complexity of phobias, which can require professional treatment to overcome for some individuals.