The answer to this question is option (d) once.
The company needs to decide once and for all what its position is on this point.
Once and for all is an idiom which means when something occurs entirely or finally, it does so once and for all.
A phrase or expression is considered to be an idiom if it usually has a metaphorical, non-literal meaning connected to it. However, some phrases keep their literal meaning while developing into figurative idioms. An idiom's metaphorical meaning differs from its literal meaning and is classified as formulaic language. Idioms are common in all languages; it is estimated that there are twenty-five million idiomatic expressions in English alone.