Final answer:
In the context of the options provided, a preconceived opinion held without reasonable support is best described as a belief, which is accepted as true without concrete evidence.
Step-by-step explanation:
An opinion that is preconceived or held without reasonable grounds of support is often referred to as a prejudice or biased opinion. In the context of the provided options, the closest term that describes a preconceived opinion without reasonable support would be belief. Essentially, a belief is something one accepts as true without needing concrete evidence to back it up, whereas a fact is a statement that can be proven true or false, a hypothesis is a testable prediction usually based on some evidence, and a theory is a well-substantiated explanation of some aspect of the natural world that is based on a body of facts that have been repeatedly confirmed through observation and experiment.