Yellowstone national park is the world's classic location for hydrothermal activity and the river drainage basin contains most of the hydrothermal activity. so, this river basin is the world's best spot to observe hydrothermal activity.

Firehole, is the right answer.

Explanation:

With its location in Wyoming and small parts of Montana and Idaho, the Yellowstone National Park is the national park of the United States of America. This national park was founded by the Congress and signed into law by the then President of the United States, Ulysses S. Grant on 1st March 1872. It has been regarded as the first national park of not just the United States but also the World. The Yellowstone National Park is well known for its wildlife and its many geothermal characteristics, particularly Old Faithful geyser.

d. i assume the underlined word is "role." reformat if it isn't.

explanation:

role is closest to part because when replacing words, "playing a vital decision," "playing a vital activity," and "playing a vital dollar" don't make logical sense. the only meaningful choice is "playing a vital part."

this fits one of merriam-webster's definitions of role: "a function or part performed especially in a particular operation or process."