Which of the following characteristics to define a hominin?
a-long upper arms
b-large brain size
c-large molar teeth
d-arboreal quadrupedalism

The correct answer is option B-large brain size.

Explanation:

Hominins are the living being which is a type or tribe of a group of an organism called primates. These primates are also called hominids.

Hominin represents the living group of the organism which shows three important characteristics:

1. Bipedalism: the ability to walk on two legs.

2. Large cranial size: large brain capacity (1500 cc) of Homo sapiens.

3. Reduced canine size: reduced size of teeth.

The only living hominin is "humans".

Thus, option B-large brain size is the correct answer.