Final answer:
The inference that the character grinning as his boy ran the bases is a father is an inference of agency, demonstrating understanding of who is behind the action described.
Step-by-step explanation:
When we read that a character grinned as his boy ran the bases, and we infer that the character is a father, this is an inference of agency. Inferring agency means understanding who is behind an action or who is participating in the action described. In this case, the use of the possessive pronoun "his" suggests a familial relation, and the context of a child playing a sport like baseball while an adult looks on proudly usually suggests a parental figure, hence the inference that the character is the boy's father.