Final answer:
The linguistic skill that demonstrates a child has developed a theory of mind is their ability to express their internal states, such as emotions or desires, through language. Option d) Saying 'I whistle because I am happy' is an example of this skill.
Step-by-step explanation:
The linguistic skills that provide solid evidence that a child is developing a theory of mind are those that reflect an understanding of others' perspectives and emotions. Specifically, the ability to use language to express internal states, such as feelings or desires, gives a clear indication of this developmental milestone.
Among the options provided, saying, "I whistle because I am happy" most directly demonstrates a child's understanding that one's actions can be caused by internal thoughts and feelings, evidencing the acquisition of theory of mind. This ability moves beyond the mere production of words, cooing, or constructing grammatically correct sentences and shows children's capacity to grasp the mental states that drive behavior—a key aspect of theory of mind.