Final answer:
The poem that best fits the theme of the infidelity of a woman and her lack of pity for her deserted lover is 'When We Two Parted' by Lord Byron.
Step-by-step explanation:
The poem that best fits the theme of the infidelity of a woman and her lack of pity for her deserted lover is B. 'When We Two Parted' by Lord Byron. In this poem, the speaker is betrayed by his lover, and he expresses his pain and resentment towards her indifference. The theme of infidelity and the absence of empathy are evident throughout the poem.