(a) Based on the scatter diagram and the correlation coefficient \( r = -0.204 \), the relationship between calories and sugar content appears to be weak. Therefore, the answer is:
\[ \boxed{\text{A}} \]
(b) The correlation coefficient between calories and sugar content is:
\[ \boxed{r = -0.204} \]
(c) The critical value for the correlation coefficient is:
\[ \boxed{r_{\text{critical}} = 0.8783} \]