To solve this problem, let's analyze the requirements for the 3x3 grid. We are given that:

• All rows, all columns, and both main diagonals of this grid must add up to the same sum.
• The grid currently contains the numbers 2, 4, and 5, along with a question mark (?) representing the unknown number we need to find.

Let's break down the solution step-by-step:

1. Assign Variables to Each Position in the Grid:

   Let’s label the cells as follows, with "?" representing the unknown number we need to replace:

   \hline a & b & c \\ \hline d & 4 & f \\ \hline ? & 5 & h \\ \hline \end{array}$$

2. Determine the Required Magic Sum:

   In a typical 3x3 magic square, the sum of each row, column, and diagonal is the same. This sum can be calculated based on the total sum of all numbers in the square.

   However, in this case, we don’t know all the numbers. But we can hypothesize that the question mark might be an integer from the options given (1, 3, 6, 7, or 9) that allows each row, column, and diagonal to have the same total sum.

3. Calculate the Potential Values for the Missing Number (?):

   Testing each answer choice to see if it results in consistent sums for all rows, columns, and diagonals.

   By inspection, the answer is 3