• To understand the proper synonym we want to know the meaning of the given the word. Massacre refers to the act or an instance of killing a variety of usually helpless people under circumstances of atrocity or cruelty.
• Now, allow us to take a glance at the meanings of every of the given options one by one.
• Option ‘A’, stab, refers to the act of thrusting a knife or other pointed weapon into someone so on wounds or kills.
• Option ‘B’, slaughter, refers to the act of killing people or animals in a cruel or violent way in large numbers.
• Option ‘C’, murder, refers to the unlawful premeditated killing of 1 person by another.
  Option ‘D’, assassinate, refers to the act of killing a very important person for political or religious reasons.
  From the above options, we will see that options ‘A’, ‘B’ and ‘C’ are incorrect as their meanings don't see killing people in large numbers.
  Hence, the right answer is option ‘B’, slaughter, because it refers to an act of cruel killing in large numbers.

Hence, option B is the correct answer.