Answer:
Obviously a metaphor.
Step-by-step explanation:
The figure of speech used in the phrase "My heart was so light" is metaphor. A metaphor is a figure of speech that describes something by saying it is something else, in order to create a comparison and make the description more vivid and expressive. In this case, the speaker is not saying that their heart is literally light in weight, but rather is using the word "light" as a metaphor to convey the feeling of happiness or joy that they are experiencing.