A person commits criminal sexual abuse by performing which act against a victim who cannot understand the nature of the act?

The correct answer focuses on the nature of sexual conduct, which involves touching that can have sexual significance. In the context of criminal sexual abuse, the law distinguishes between different types of sexual offenses, and sexual conduct is defined as any kind of physical contact that may have sexual implications. This choice is particularly relevant in cases involving victims who lack the capacity to consent or understand the nature of the act, which is critical for the classification of the offense.

Sexual penetration, while also a serious offense, typically involves more invasive actions that are classified differently under Illinois law regarding sexual crimes. Sexual manipulation is not a recognized legal term in this context, making it less relevant to the definition of criminal sexual abuse.

Thus, the emphasis on sexual conduct aligns with legal definitions and considerations regarding consent and a victim's understanding, making it the most accurate choice in relation to the question posed.