Final answer:
A boastful person, being full of boasts, is most likely to brag about a grade, as they are inclined to talk with excessive pride about their achievements.
Step-by-step explanation:
The suffix -ful added to a word denotes that something is 'full of' the root word provided. In the context of a boastful person, this means someone who is full of boasts, or one who brags or talks with excessive pride about their achievements. Of the options provided, a boastful person would be most likely to C. brag about a grade.
The suffix -ful is used to form adjectives that describe someone or something as full of a particular quality or characteristic. In the context of the question, a boastful person would be most likely to brag about a grade. Boasting is a behavior associated with being proud and showing off, which aligns with the meaning of the suffix -ful.