**Core Concept:** Sexual perversion refers to abnormal or non-conventional sexual behavior or desire that deviates from the norm and causes distress or impairment in daily functioning.

**Why the Correct Answer is Right:** Frotteurism is a sexual perversion characterized by sexual arousal from observing or touching another person without their consent, particularly focusing on genitalia. This can lead to distress and interference with daily life due to the individual's obsessional thoughts and behavior.

**Why Each Wrong Option is Incorrect:**
A. Exhibitionism: Sexual perversion involving exposing one's own genitalia for sexual pleasure, typically in public places. This option is incorrect because it focuses on exhibiting one's own genitalia, not observing or touching another person without consent.
B. Sadism: Sexual perversion characterized by sexual pleasure from inflicting pain on the partner. Sadism is unrelated to observing or touching another person without consent.
C. Masochism: Sexual perversion characterized by sexual pleasure from receiving pain or suffering inflicted by the partner. Masochism is unrelated to observing or touching another person without consent.
D. Frotteurism: As explained above, the correct answer to this question. Frotteurism is characterized by sexual arousal from observing or touching another person without their consent, particularly involving genitalia.

**Clinical Pearl:** Frotteurism can coexist with other sexual perversions, making it essential for clinicians to consider this diagnosis in cases of compulsive and inappropriate sexual behaviors in patients.

**Correct Answer:** D. Frotteurism