Explanation:

- The total score for 75 students with an average of 45 is $75\times 45=3375$.

- The total score for the first 50 students with an average of 42 is $50\times 42=2100$.

- The total for the last 26 students with an average of 52 is $26\times 52=1352$.

- The sum of the first 50 and last 26 scores is $2100+1352=3452$.

- Since the total for all 75 scores is 3375, the 50th score must be $3452-3375=77$.

Correct Answer: Option 4 — 77