Step 1: List the outcomes:
E (prime): 2, 3, 5, 7. F (X less than 4): 1, 2, 3. The union \(E\cup F = \{1, 2, 3, 5, 7\}\). The outcomes 2 and 3 are in both, so they are counted only once.
Step 2: Add the probabilities:
\(P(E\cup F) = P(1) + P(2) + P(3) + P(5) + P(7) = 0.15 + 0.23 + 0.12 + 0.20 + 0.07 = 0.77\).
Step 3: Check by the formula:
\(P(E) = 0.23 + 0.12 + 0.20 + 0.07 = 0.62\), \(P(F) = 0.50\), \(P(E\cap F) = 0.35\). Then \(0.62 + 0.50 - 0.35 = 0.77\).
Final Answer:
\(P(E\cup F) = 0.77\), option (B).
\[ \boxed{0.77} \]