Hi queenbee!
Happy to address answer choices (A) and (C).
First, this a particular variant of must be true question, a principle question. In this question type, one is supposed to choose an answer choice that not only must be true based on the stimulus, it also must reflect a principle underlying the argument. The word "assessments" in the question stem indicates that this is a principle question. Noting the question type is an important starting point with regards to your concern that answer choice (C) was too vague. Vagueness doesn't necessarily disqualify an answer choice for this type of question and, to the contrary, it can often work well in terms of abstracting a broader principle or assessment that must be underlying the stimulus.
Answer choice (C) states, "Changes in people's feelings about life can result from technological changes." You note about this answer,
"Changes in peoples feelings about life" could be a good/positive change too. It seemed to me that the critic was saying that technological advancements have led to instability and impermanence, which is negative.
You're right in noting that the language of "changes in people's feelings about life" might indicate positive or negative changes. But that alone makes it work well as the right answer choice, rather than disqualifying it, for the broader, more encompassing language includes positive, negative, or other types of changes. You also raise a good point that the particular example of change in the stimulus seems to be negative inasmuch as it creates "feelings of impermanence and instability." Again, though, the more encompassing language of (C) includes these feelings. A differently worded answer choice (C) might seem more implausible if it only referred to positive feelings.
That explains why answer choice (C) is correct. With respect to answer choice (A), it states, "The fast pace of modern life has made it difficult for people to achieve their goals." This answer choice can be eliminated because its phrased in terms of difficulty, but difficulty isn't mentioned in the stimulus. We're told that "This speed has created feelings of impermanence and instability, making us feel as if we never have enough time to achieve what we want-or at least what we think we want." Based on this information alone, we don't know one way or another about difficulty. In addition, one might argue that the lack of time suggests difficult, but this raises another ambiguity in the answer choice. It refers to "goals," which also aren't specifically mentioned. Does "goals" refer to what we want? Or what we think we want? Answer choice (A) raises elements that we don't know for sure based on the stimulus, making it incorrect given this must be true/principle type of question stem.