The correct answer is B) H⁺.
When an atom loses one electron, it becomes positively charged due to the imbalance between the number of protons (positively charged) and electrons (negatively charged) in the atom. In the case of the options given:
- A) Ca²⁺ indicates that a calcium atom has lost two electrons.
- B) H⁺ indicates that a hydrogen atom has lost one electron, resulting in a single positive charge.
- C) K⁺ indicates that a potassium atom has also lost one electron, but it is not the only answer; however, it does indicate a single positive charge as well.
Thus, both B) H⁺ and C) K⁺ indicate loss of one electron, but since the question implies an individual selection, H⁺ is often the most straightforward example of an atom that has lost one electron.