The smallest puppy in the litter is energetic this morning is the subject complement in the sentence a noun pronoun or agjective

It's an adjective; "The smallest puppy" is the subject of the sentence as the rest of the sentence describes it. Of course the subject cannot just be "puppy" so it must include "The smallest" as well. Since smallest is an adjective (A word that describes something else,) that answer is Adjective.