Step 1: Understand the nature of a “must be true” question.
A statement must be true only if it logically and necessarily follows from the given facts, without adding assumptions.
Step 2: Analyze Statement I.
There is no information about the total seating capacity of the festival. Hence, we cannot conclude that any seats will remain empty. This statement does not necessarily follow.
Step 3: Analyze Statement II.
There is no mention of the performers’ family members in the given facts. Their presence at the festival cannot be inferred. Hence, this statement is not necessarily true.
Step 4: Analyze Statement III.
Although ticket sales have been ongoing and preparation started 10 weeks ago, there is no guarantee that all tickets will be sold by Monday. This is an assumption, not a fact.
Step 5: Final conclusion.
None of the given statements must necessarily be true based on the provided facts alone.
Final Answer:
\[
\boxed{\text{Option (E)}}
\]