What is the formula of the ion formed when potassium achieves noble-gas electron configuration

Potassium is an alkali metal which means that it only needs to get rid of one electron to achieve a noble gas configuration.  Since it only needs to get rid of one electron the ion will be a cation with a charge of +1 which means the symbol will be K⁺.

I hope this helps.  Let me know if anything is unclear.

There is no reason to think the results are biased or not conducted properly but you should not assume the results are correct just because they were done in a laboratory so the answer would be c. it is likely to be reliable because the results support the conclusion presented.