In a grid puzzle, each row and column in the 9x9 grid, as well as each 3 × 3 sub-grid shown with heavy borders, must contain all the digits 1 - 9.

In the above partially filled grid, the square marked “?” contains  

Correct option is C

Solution:
As per the rule in each row, column and subgrid must contain all the digits 1-9 once. If we look at nearby subgrids and the options, 9 cannot occur at any other place than at question mark. 

If we took at the 2nd  3x3 grid, 6 can't be presented in the topmost row and the middle row, as in those entire rows, 6 is already present.
In bottom row of same grid, 6 cannot be present in 1st and 2nd box as 6 is already present if we check that entire column.
In this 2nd  3x3 grid, 6 is not present, Hence the question marked box has to be 6.
Hence, Option (c) is true.