Final answer:
The act of section 18 causing GBH (with intent) refers to intentional infliction of grievous bodily harm.
Step-by-step explanation:
The act of section 18 causing GBH (with intent) refers to intentional infliction of grievous bodily harm. This is option a) in the given choices. Intentional infliction of grievous bodily harm means that a person has purposely caused severe physical harm to another individual.