Answer : 1. "vindictive"
Explanation :

The apparition (=ghost) that stalked Reshma for days, ultimately threw her off the cliff.

So, ‘benevolent’ and magnanimous’ won’t fit the blank as these are positive words.“Benevolent” means well-meaning and kindly

“Magnanimous” means generous or forgiving, especially towards a rival or less powerful person.

“Callous” means indifferent/ showing or having an insensitive disregard. It does not make any sense in the given context, and hence, can be eliminated

Only ‘vindictive’ fits the blank appropriately.“Vindictive” means having or showing a strong or unreasoning desire for revenge/ revengeful