Th. It can be anything, ranging from your parents, some relative, your school teacher, or even your curiosity towards the current changes in the environment etc. Whatever is your reason, be honest about it and inform it to the interviewer in a nice positive manner.
Sometimes, money is also our motivation behind making a career choice but this is not something that you might want to use for this answer. So, use something that sounds unique and assures the interviewer about the fact that you are in this profession to make a change and not just for the money.
"I want to become an environmental scientist because I have always been curious about the recent changes in the environment and I wanted to do my share of duty towards improving the situation as much as I can."
"I grew up with both parents as a scientist, and my mother specifically was an environmental scientist. She was so passionate about her work that it inspired me to follow in her footsteps. I recently graduated with a Bachelor of Science in Environmental Science and look forward to starting a long and fruitful career with your organization."
"I have carved a very successful career for myself, within environmental science, over the past decade. After graduating high school, I spent one year traveling to a variety of undeveloped countries. This trip changed the way that I thought about the way we consume and pollute in our more developed countries. This was when I decided that I could make a difference by working in environmental science."