One should never be reckless in his dealings with others.

Answer: Option B

Reckless : heedless of danger or the consequences of one's actions; rash or impetuous.
Indifferent : having no particular interest or sympathy; unconcerned.
Careful : making sure of avoiding potential danger, mishap, or harm; cautious.
Imprudent : not showing care for the consequences of an action; rash.
Lenient : (of a punishment or person in authority) more merciful or tolerant than expected.

Antonym of Reckless is Careful