Answer:
The correct answer is: "irony".
Step-by-step explanation:
Irony is the stylistic element used in these lines because they really are the contrary of what they only seem. "Give back your heart to itself", that is, to your own heart. Therefore, the irony is built by different meanings, by the sentences "to the stranger who has loved you all your life", and someone's own heart is not actually a stranger; and by "who you ignored for another, who knows you by heart", which is an obvious statement since someone's own heart should be known by itself instead of someone else's. The other options are not possible in this case because they are lacking stylistic elements.