Hey, the eligibility criteria for JNU MSc Environmental science course are

Candidates must have a Bachelor’s Degree in any branch of Basic or Applied Sciences under 10+2+3 education pattern from school/ Institute/ University as recognised at the state or national level.

Candidates need to have secured at least 55 percent marks in aggregate of subjects of qualifying degree examination.

So because you are from an arts background, this may not be applicable to  you

Best wishes