Answer:
2) Courage.
Explanation:
A connotation can be defined as the quality, feeling or an idea which a word brings to the mind of a person, as well as its literal, dictionary or primary meaning.
Courage refers to the quality that is being possessed by a confident or brave individual which makes him or her not to be intimidated, frightened easily while being cautious in his or her actions.
Hence, the word "courage" provides an appropriate connotation of the word heart as it is used in this sentence: The guard dog defended his owner with the heart of a warrior.
This ultimately implies that, the dog defended his owner courageously like a warrior.
