The correct option is
A Elegy
Option A - 'Sonnet' is a poem that has fourteen lines and has a particular pattern of rhyme. Thus option A is incorrect.
Option C - 'Lyric' is a short poem that expresses the personal thoughts and feelings of a person who wrote it. Thus option C is incorrect.
Option D - 'Epic' is a long narrative poem about the heroic deeds and events that are significant to the culture of the poet. Thus option D is incorrect.
Option B - 'Elegy' is a sad poem or a song. Thus option B is the correct answer.
NOTE: 'Mourning' means - sadness
'A poem of mourning - Elegy'