Ignoring the digits, every term is built from the same three letters in the same order, Q, then R, then T, with one digit inserted somewhere among them. Reading the digits across the series, 4, (missing), 6, 7, 8, they simply count upward by 1 each time, so the missing digit must be 5. Reading where that digit sits, term 1 'Q4RT' has it right after Q (position 2), term 3 'QRT6' has it at the very end (position 4), term 4 'QR7T' has it just before the final T (position 3), and term 5 'Q8RT' has it back after Q (position 2), the position swings 2, 3, 4, 3, 2, like a pendulum. So the missing second term needs the digit 5 sitting at position 3, giving Q, R, 5, T. Let's check the options against this:
Only option B preserves the Q-R-T letter order and continues the digit sequence and position pattern correctly.
Therefore, the correct answer is QR5T.
Numbering the five terms 1 through 5 (Q4RT, ?, QRT6, QR7T, Q8RT), two independent formulas describe the pattern. First, the inserted digit equals the term number plus 3: term 1 gives \(1+3=4\), term 3 gives \(3+3=6\), term 4 gives \(4+3=7\), term 5 gives \(5+3=8\), so term 2 must carry the digit \(2+3=5\). Second, the position of that digit among the four characters follows \(4-|n-3|\), where n is the term number: term 1 gives \(4-|1-3|=2\), term 3 gives \(4-|3-3|=4\), term 4 gives \(4-|4-3|=3\), term 5 gives \(4-|5-3|=2\), so term 2 must place its digit at position \(4-|2-3|=3\). Combining both formulas, the missing term is the letters Q, R, T with the digit 5 inserted at position 3, that is Q, R, 5, T.
Only option B satisfies both the digit formula and the position formula for term 2.
Therefore, the correct answer is QR5T.