which

the company which

Interessant wird die Sache, wenn "Firma" mit Tätigkeiten in Verbindung gebracht wird:

Die Firma/Mercedes hat beschlossen, in Amerika weiter auf Expansion zu setzen.

The company/Mercedes has decided to ...
The company/Mercedes have decided to ...

Ist hier nicht beides möglich, so wie bei
"The Board was/were preparing to ...." oder
"The chorus was/were given its/their costumes"?

principally you can use plural *or* singular in so-called nouns of number - 'multitude', for instance, and the choice is regulated by 1/ popular choice; 2/personal preferences; 3/ context

so, in the examples offered, both forms are possible

Fowler has this to say on whether singular or plural in an example using 'Cabinet':

" Nouns of multitude may stand either for a single entity or for the individuals who compose it. They are treated as singular or plural at discretion - and sometimes, naturally, without discretion. 'The Cabinet IS divided' is better, because in the order of thought a whole must precede a division; and 'The Cabinet ARE agreed' is better, because it takes two or more to agree. That is a delicate distinction, and few will be at the pains to make it."

Great explanation, odondon. Which Fowler edition is it that you quoted?

I take it from what you said that in Rhino's example you prefer the singular "which", because it refers to a state of affairs, while in my examples the plural is better, because they refer to actions that imply people who are acting.

I think it necessary to split up the discussion into the two questions being raised.
question 1 is: do we consider a company to be a person or a thing?
if it is a person, then it is correct to write: 'the company who'
if it is a thing, then it is correct to write: 'the company which'.

in the question asked originally by Rhino, there is not enough context to decide whether the author is referring to the company as thing or person.
in doubt I always go for the word itself, and this refers to a thing.
that is why I suggested 'which' as the proper word.

question 2 is: what form of verb follows words of multitude?
the answer to this is given by Fowler (first edition, 1926 - lightly paraphrased by me). Here it is pretty immaterial whether 'which' appears or not, decisive is the context which allows us to choose whether we consider a company to be a single unitary entity, and let it act as singular; or whether we consider it to be a collective of parts and force it to be a plural - for this decision I refer to Fowler.