The correct answer is 7.

Solution:
Consider the given equation.
4 + 4 = 15 –

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

Which gives:
4 + 4 = 8

So,

4 + 4 = 15 –

8 = 15 -

Next, we look for the number which when subtracted from 15 gives 8.

Note that,
15 - 7 = 8

Then,
4 + 4 = 15 – 7

So, number 7 makes the equation true.