When is a German employment contract binding, if it does NOT mention an...

[ordinary guy]

...Effective Date? When is a German employment contract binding, if it does NOT mention an Effective Date?

Which of the following is the Effective Date:

a) on the day when the employer signs both original copies?
b) several days later, after the posted contracts arrive at the employee's house (in another city)?
c) several days later, when the employee signs the contracts, just before posting them to the employer?
d) when the signed original contract arrives back at the employer’s location?

If it is (d), does this mean that the employer can withdraw the offer/contract, even if he has signed it?

Please assume that there is approximately one week in bewteen each stage, i.e. 3-4 weeks between (a) and (d).

Thanks for your advice!