In my humble opinion, execution is never "humane".
What is "humane"?
According to Webster
mid-15c., a parallel variant of human (adj.), with a form and stress that perhaps suggest a stronger association with Latin humanus than with Old French humain.
Human and humane were used interchangeably in the senses "pertaining to a human being" and "having qualities befitting human beings" (c. 1500). The latter at first meant "courteous, friendly, civil, obliging," then "marked by tenderness, compassion, and a disposition to kindly treat others kindly".
Compare germane, urbane. Meaning "inflicting less pain than something else" is from 1904. Inhuman is its natural opposite. The Royal Humane Society (founded 1774) was originally to rescue drowning persons; such societies had turned to animal care by late 19c.
What stands out to me in the definition is "marked by tenderness, compassion".
So, according to your question and the definitions afforded by Webster, none of your provided options are "humane".