The correct answer is D. sabotage.
Sabotage is the act of deliberately damaging or destroying something, especially for political or military reasons. In this case, the bombs exploded in five trains within a short span of time, which suggests that it was a deliberate act of sabotage. The perpetrators of the sabotage may have been trying to disrupt the transportation system, or they may have been trying to send a message to the government.
The other options are not as likely. A game is something that is played for amusement or competition. A conspiracy is a secret plan by a group of people to do something harmful or illegal. Villainy is the quality of being wicked or evil. While it is possible that the bombs exploded in five trains within a short span of time as part of a game, a conspiracy, or an act of villainy, the most likely explanation is that it was an act of sabotage.