Final answer:
The ion formed when potassium achieves a noble-gas electron configuration is K+, which is a singly positive charged ion or cation.
Step-by-step explanation:
The formula of the ion formed when potassium achieves noble-gas electron configuration is K+. Potassium has an atomic number of 19, which means it has 19 protons and, when neutral, 19 electrons. Having just one electron in its valence shell, potassium tends to lose that single electron to achieve the electron configuration of argon, a noble gas. This loss of electron results in a potassium ion with a positive charge of +1, because there are now more protons than electrons in the ion. Therefore, the correct answer is c) K+, indicating a singly positive charged ion or cation.