Which suffix would you use to indicate being full of care?

The suffix that indicates being full of care is "-ful." This suffix is used to form adjectives that describe someone or something as having a particular quality. When added to the word "care," it creates the word "careful," which directly means full of care or exercising caution.

In contrast, the other suffixes do not convey the same meaning. The suffix "-ness" typically indicates a state or quality, such as in "happiness." The suffix "-less" implies the absence of a quality, as seen in "careless," which means lacking care. Lastly, the suffix "-ly" often transforms adjectives into adverbs, as in "quickly," which describes how an action is performed, rather than conveying the quality of being full of care. Thus, "-ful" is the best choice here for expressing the desired meaning.