Final answer:
The factor most associated with a significant increase in depression and anxiety among American teenagers is social media usage. However, physical activity is known to reduce these mental health issues while sedentary behaviors are linked to an increased risk of anxiety. Moreover, depression can adversely affect educational achievements due to its impact on cognitive functions.
Step-by-step explanation:
The question that asks what has seen a gigantic increase in depression and anxiety for American teenagers can be attributed to multiple factors, but the most relevant based on current research seems to be B) Social media usage. Although not one of the provided options, extensive studies and anecdotal reports have highlighted the negative impact that social media can have on mental health, especially among adolescents. It feeds into issues of self-esteem, comparison, cyberbullying, and can lead to increased feelings of isolation despite being more 'connected' than ever.
Nevertheless, it's important to note that physical activity is often linked to reduced levels of depression and anxiety. Physical exercise is beneficial for mental health because it reduces stress, improves body image, and promotes positive self-esteem. Sedentary behaviors, which have become more common due to the increased time spent on activities like web surfing and gaming, are associated with an increased risk of anxiety. It's been found that regular physical exercise can be as effective as antidepressant drugs in the treatment of depression and can offer several other health benefits such as better cardiovascular health and cognitive function.
While not directly addressed in the options, it is understood that mental health issues such as depression can also negatively affect educational achievements, as they can impair memory, focus, and decision-making abilities, which are crucial for learning and academic performance. On the other hand, outdoor recreation, like physical activity, has positive effects on mental health and could potentially buffer against anxiety and depression.