Explanation:

Definition of 'culpable':
The word 'culpable' is an adjective that describes someone or something as deserving blame or censure for a wrongdoing or mistake. It implies that the person or thing is responsible for their actions and should be held accountable for them.

Options:
a) Defendable: This option is incorrect as 'defendable' means capable of being defended or justified, which is the opposite of culpable.
b) Blameless: This option is correct as 'blameless' means without fault or guilt, which is the opposite of culpable.
c) Careless: This option is incorrect as 'careless' means not giving sufficient attention or thought to avoiding harm or errors, which is different from being culpable.
d) Irresponsible: This option is incorrect as 'irresponsible' means not showing a proper sense of responsibility, which is different from being culpable.

Explanation of the correct answer:
The correct answer is option 'B' - Blameless.
When someone is described as 'culpable', it means they are responsible for a wrongdoing or mistake. Therefore, the opposite of culpable is 'blameless', which means without fault or guilt. If someone is blameless, they are not responsible for the actions or consequences in question.

Example sentence:
- Despite the evidence, the defendant was found blameless and acquitted of all charges.
- The blameless driver was a victim of the reckless actions of others on the road.

In conclusion, 'culpable' means deserving blame or censure, and the opposite of culpable is 'blameless', which means without fault or guilt.