This is a ............... on his character.

Answer: Slur

Explanation: "Slur" means a defamatory or insulting remark that damages someone's reputation. The sentence "This is a slur on his character" correctly uses "slur" to indicate a harmful statement about his character.

• Blot — means a stain or blemish. Although "a blot on his character" can describe a stain on reputation, it usually refers to a lasting blemish rather than a spoken defamatory remark.

• Blur — means to make unclear or indistinct; it does not fit the meaning or collocation in this sentence.

• Spot — refers to a small mark or to notice something and is not used with "on his character" to mean a damaging remark.