The correct answer is $\boxed{\text{D}}$. The hydronium ion is a positively charged ion formed when water molecules react with hydrogen ions. It is written as $H_3O^+$ and is often called the oxonium ion. The hydronium ion is the most common form of hydrogen ion in aqueous solution. It is responsible for the acidity of water and other solutions.
Option A, $H^+$, is the hydrogen ion. It is a bare proton, which is a positively charged subatomic particle. The hydrogen ion is very reactive and does not exist in solution by itself. It is always bonded to another molecule, such as water, to form a hydronium ion.
Option B, $HO^-$, is the hydroxide ion. It is a negatively charged ion formed when water molecules react with hydroxide ions. It is written as $OH^-$ and is often called the hydroxyl ion. The hydroxide ion is the most common form of hydroxide in aqueous solution. It is responsible for the basicity of water and other solutions.
Option C, $H_2^+$, is not a valid chemical formula. It is not possible for two hydrogen atoms to bond together to form a positively charged ion.