If the posted speed is not safe at that time
a & b
Whatever the posted speed limit is +or- 5 mph.
The maximum speed limit.
Generally 5 mph but you should always check the particular parking area for the posted speed limit.
Only if weather or road conditions safely allow it.
The posted speed limit is the top speed in GOOD weather and visibility. Bad conditions, slow down- or get a ticket for "Unsafe speed for conditions".
The posted speed limit is the max speed.
the posted speed limit
False. It's usually a great idea to drive a lot slower than the posted speed limit when the weather is bad, because if you drive at the posted speed limit in bad weather, you are much more likely to lose control of your vehicle and crash. The posted speed limits are for good weather conditions, not bad weather conditions.