The correct answer is 3.

Solution:
Consider the given equation.
4 + = 10 – 3

To determine the missing number, we need to find: 10 – 3.

Which gives:
10 - 3 = 7

So,
4 + = 10 – 3

gives:

4 + = 7

Next, we look for the number which when added to 4 gives 7.

Note that,
4 + 3 = 7

Then,
4 + 3 = 10 – 3

So, number 3 makes the equation true.