In which of the following is the number of electrons the ion contains given correctly

B. K+ has 18 electrons

Explanation:

Ions are formed when atoms of element gain or loose electrons.When an atom looses electrons it forms a positively charged ion and when an atom gains electrons it forms a negatively charged ion.Metal atoms such as potassium, magnesium and aluminium looses outer most electrons to positively charged ions.Potassium atom has 19 electrons, 19 protons, with an electronic configuration of 2 : 8 : 8 : 1. To form an ion, K looses an electron to form a positively charged potassium ion (K+) with 18 electrons and 19 protons and an electronic configuration of 2:8:8.

Gallium melts near room temperature and has one of the largest  liquid  ranges of any metal, so it has found use in high temperature thermometers.