The correct option is
A Considerate
The word 'callous' refers to someone having or showing an insensitive and cruel disregard for others.
Option A, considerate refers to someone who is careful not to inconvenience or harm others.
Option B, indifferent refers to someone who has no particular interest or sympathy.
Option C, indulgent refers to having or indicating a readiness or over-readiness to be generous or lenient with someone.
Option D, generous refers to showing a readiness to give more of something than is strictly necessary or expected.
Thus, according to the meaning, the word 'considerate' is the most opposite in meaning from the word 'callous'.