Correct Answer: heedless

Solution : The correct choice is the third option .

Discreet means careful and cautious. Heedless means not paying attention, the opposite of being careful. Hence, heedless is the antonym.

The meanings of the other options are as follows:

• Careful : Taking care to avoid mistakes or harm; exercising caution.
• Harmless : Not causing harm; safe.
• Tactful : Having or showing tact, which is the ability to deal with others in a sensitive and considerate way, avoiding offence.