Which of the following is necessary to form a valid hypothesis?

To form a valid hypothesis, existing knowledge relevant to the query is crucial. This foundational knowledge helps ensure that the hypothesis is grounded in evidence and scientific understanding, making it more robust and testable. A hypothesis should be based on what is already known in the field, allowing researchers to make informed predictions about outcomes. This context is essential for framing the inquiry appropriately and enabling accurate analysis and interpretation of results.

The other options do not provide the necessary basis for forming a valid hypothesis. Observing an unrelated phenomenon lacks the specificity and relevance required, while a broad assumption unsupported by evidence fails to offer the specificity dictated by existing knowledge. A personal opinion on the subject is subjective and does not provide a scientifically valid starting point for forming a reliable hypothesis.