The correct answer is the following: option d. The speakers in Emily Brontë's "Remembrance" and Thomas Hardy's "Ah, Are you Digging on My grave?" and "The Darkling Thrush" have in common that all three share feelings of sadness and loss.
In "Remembrance" the speaker is reflecting on her lover's death, and how this one has impacted and changed her life. In "Ah, Are you digging on My grave?" the speaker is dead and buried and it is in her grave that the whole poem sets place. Finally, in "The Darkling Thrush" the speaker describes the dying of the old world and how he can not be happy with the replacement of the new one. All three speakers are dealing with emotions of sadness and loss, and are describing their ways and mechanisms of coping with those emotions.