Step 1: Understanding the Concept:
For two events, \(P(E \cup F) = P(E) + P(F) - P(E \cap F)\).
Step 2: Find the events.
\(E\) (prime numbers in 1 to 8): \(\{2, 3, 5, 7\}\). \(F\) (\(X < 4\)): \(\{1, 2, 3\}\). \(E\cap F = \{2, 3\}\).
Step 3: Add the probabilities.
\(P(E) = 0.23 + 0.10 + 0.20 + 0.07 = 0.60\).
\(P(F) = 0.15 + 0.23 + 0.10 = 0.48\).
\(P(E\cap F) = 0.23 + 0.10 = 0.33\).
Step 4: Combine.
\[ P(E\cup F) = 0.60 + 0.48 - 0.33 = 0.75 \]
Final Answer:
\(P(E\cup F) = 0.75\), option (D).
\[ \boxed{0.75} \]