Would like to have passive income with savings of $300k. Where would be best?

If you want the theoretical answer, risk free rate = 10-year US treasury bond. That's the risk free rate on capital asset pricing model (CAPM). But in reality, as what others pointed out, nothing is risk-free, is just how much of a risk you want to take.

An investment must have exposure to risks in order to have a return (dividend, capital gains, etc). Therefore I would say that is not possible.

Technically even the most negligible risks also has risk - so zero risk is actually a myth. The closest you can get to that is government bonds, or probably your own bank savings account. You can consider to diversify in STI ETF, bonds or CPF.

Not investing also got inflation risk lol, put CPF also got policy change risk. In life, you cant avoid risk, you can only seek to minimize risk