Final answer:
The argument is true. The defendant's mental state played a crucial role in the jury's verdict of not guilty. The correct option is a.
Step-by-step explanation:
The argument, 'The defendant is not guilty of murder since she is insane,' is true.
In this case, the defense claimed that the defendant had the mental capacity of a ten-year-old child and did not understand the nature of her actions or the enormity of the crime she committed.
The jury accepted this argument and found the defendant not guilty. This highlights the importance of the defendant's mental state in determining their guilt or innocence in a murder trial.
Hence, Option a is correct.