The social or physical characteristics that mark the boundaries between groups are highly

a. Highly visible
b. selected by the minority groups themselves
c. Scientifically significant
d. Selected for their biological improtance
e. subtle

a. Highly visible

To determine the correct answer among the options provided, let's break down each option and analyze its relevance.

a. Highly visible
This option suggests that the characteristics marking group boundaries are easily noticeable. While this may be true in some cases, it is not necessarily always the case. Boundaries can also be defined by subtle characteristics that are not immediately visible.

b. Selected by the minority groups themselves
This option implies that the characteristics defining group boundaries are determined by the minority groups themselves. While this may be a factor in some cases, it doesn't necessarily apply universally to all groups.

c. Scientifically significant
This option suggests that the characteristics marking group boundaries have scientific significance. While social and physical characteristics can be studied scientifically, it does not necessarily mean that they are universally significant in all contexts.

d. Selected for their biological importance
This option implies that the characteristics defining group boundaries are chosen based on their biological importance. However, defining group boundaries can encompass a range of factors beyond biological significance.

e. Subtle
This option states that the characteristics marking group boundaries are subtle. This could be a plausible answer since boundaries between groups might be defined by nuanced or hidden attributes that are not immediately apparent.

Based on the analysis above, the correct answer is most likely "e. subtle." This option acknowledges that the social or physical characteristics marking group boundaries can often be subtle and not easily observable.