Steady Source of Income for Parents

What's one (or maybe two) to ensure they keep getting steady income for next 15-20 years.

Inflation and everything aside.. but just generate like 10-20K per month for them from a corpus of say maybe 5-10 Lakhs?

but no waiting period and they can start immediately.. any govt schemes?

it's ohkay if over the period money doesn't grow but it should generate regular income for them to cover basic expenses.

Any suggestions would be great.