When solving for a variable, you want to get it isolated (i.e. alone). Here are the steps to do this equation...
1. Given: kx-3=5
2. Add three to both sides to remove the -3, and this gives you kx=8
3. Since the x is being multiplied by k, you want to do the inverse and divide both sides by k, and this gives you x=8/k
So this means that your answer is D
If you need any further explanation, let me know :)