Stop and try to find out the speed. – This feels like an overreaction that may do more damage than good, for example, by obstructing other vehicles.
Assume I may not drive faster than 50 km/h. – This is in line with the quotation from the Road Traffic Ordinance. Note that this option does not say that you must drive at 50 km/h, but rather that this is the upper limit!
Follow the base speed limit which 70 km/h. – Wrong, 70 km/h is the base speed limit outside a built-up area.
Drive at around 10 km/h until I see the next speed sign. – This feels like an overreaction that may do more damage than good, for example, by obstructing other vehicles.
Another way of looking at it: There are both road signs and regulations that indicate maximum speed limits. You shall firstly adhere to the road signs, but if you have forgotten these it is reasonable to “fall back” on the regulations, which state that a maximum speed limit of 50 km/h applies within a built-up area. This is much more reasonable than panicking and braking dangerously.