Answer:
The score on the remaining test is x = 61
Hence, option D is correct. 
Explanation:
Let 'x' be the score of the remaining test
The average score of Nadine's six tests = 77
so
Average score = (88+65+75+83+90+x) / 6
substituting average score = 77 in the equation to find 'x'
 77 = (401 + x) / 6
 401 + x = 77 × 6
 x = 462 - 401
 x = 61
Therefore, the score on the remaining test is x = 61
Hence, option D is correct.