The question is about determining the minimum slope for an accessible ramp as per most building codes. The options provided are different slope ratios.

Let's analyze each option:

• 1 : 12 - This indicates that for every 1 unit of vertical rise, there is a 12-unit horizontal run. This ratio is commonly recognized as the maximum slope for ramp accessibility according to many building codes, including the Americans with Disabilities Act (ADA) and often referred in Indian building standards as well.
• 1 : 25 - This slope is much gentler compared to 1:12. It is not the standard ratio for minimum slope in building regulations regarding accessibility.
• 1 : 15 - This slope is between 1:12 and 1:25 and also is of a gentle gradient but generally not the minimum recommended slope.
• 1 : 20 - Again, this is a gentler slope compared to 1:12 although sometimes referenced for other purposes like walking slopes but not for wheelchair accessibility.

Among the options given, a slope of 1 : 12 is generally considered the standard minimum for accessible ramps as per many building codes. Hence, the correct answer is 1 : 12.