A simile is a figure of speech that presents the likeness or draws a comparison between two unlikely things and contains the words 'like' or 'as'. For example, as smooth as butter, light like a feather and so on. In the given sentence, a comparison is made between the person, Ruchi, and a deer, as deer are known for their speed and quickness. The phrase "ran like a deer" contains 'like', providing further proof that this phrase is a simile, hence option D is correct. "Leopard" is a noun, it's not a simile as it does not compare any two things, hence option A is incorrect. "Came out of nowhere" is a phrase made up of a verb + adverb + preposition + adverb, it does not compare anything, hence option B is incorrect. "To attack Ruchi" is a phrase, made up of an infinitive (to attack) and a noun (Ruchi); it's not a simile as it doesn't compare anything, hence option C is incorrect.