Stereo selective reaction products Pyruvic acid on reduction with yeast gives (-) lactic acid while with $\ce{NaBH4}$ it reduces to (+) lactic acid. How?


I know this has something to do with stereoselectivity. But I don't know which one would be a correct answer.

It would be more helpful if you could explain a bit about stereo-selective reactions. I know what it is, what I want to know is, how to find which of the products will be stereochemically preferred one, just like in this question.
Enyzmatic reduction (as would occur in yeast) could easily be a stereoselective process - probably via selective delivery of a hydride from NADH.

As the other respondent notes, there is simply no way that combining achiral reagents (pyruvic acid and NaBH4) can yield anything other than a racemic product.