The correct answer is 1.

Solution:
Consider the given equation.
4 + 5 = 10 –

To determine the missing number, we need to find: 4 + 5.

Which gives:
4 + 5 = 9

So,

4 + 5 = 10 –

9 = 10 –

Next, we look for the number which when subtracted from 10 gives 9.

Note that,
10 - 1 = 9

Then,
4 + 5 = 10 - 1

So, number 1 makes the equation true.