Hello Vihaan,
Yes, The National Institute of Open Schooling (NIOS) is valid in Mumbai University. Here are the eligibility criteria for admission in mumbai university through NIOS -
http://mu.ac.in/portal/wp-content/uploads/2014/03/upload-eligibility-instruction.pdf
The above document is not just for NIOS, but it also includes NIOS.
Hope this helps.
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