How to calculate the concentration based on dilution of a stock solution?
Well, I'll start by saying I redid your math, and you have rounding errors.  Using correct sig figs, your stock solution has 100. mg of Li in it, giving a concentration of 1.00E3 mg/L.  Those dilutions therefore should be 30.0 mg/L, 50.0 mg/L, 80.0 mg/L, and 100. mg/L.

You're correct aside from the rounding errors, and the answer key is wrong.  The question as you've written can't have those answers, as 300 μg/L is 0.3 mg/L, which is not the answer.