Final answer:
Hydroxide ions (OH-) will form a precipitate with calcium ions (Ca2+) as calcium hydroxide (Ca(OH)2), which is insoluble in water.
Step-by-step explanation:
The anion that will form a precipitate with Ca2+ among the options given is OH-. When calcium ions (Ca2+) react with hydroxide ions (OH-), they form calcium hydroxide (Ca(OH)2), which is an insoluble substance and, therefore, precipitates out of the solution. This is in accordance with the solubility rules of common ionic solids in water. The compounds formed with Cl- (calcium chloride, CaCl2) and C2H3O2- (calcium acetate, Ca(C2H3O2)2) are generally soluble and will not form a precipitate with calcium ions under typical conditions.