Correct Answer: cruel

Solution : The correct choice is the fourth option .

Heartless is an adjective that means lacking compassion.

Cruel refers to actions that cause pain and suffering and to show no regard or compassion towards others. Hence, it is the correct synonym for the given word.

The meanings of the other options are as follows:

• Robust means strong and healthy.
• Warm means friendly and affectionate.
• Mild means something that is not severe.