O2(g) → o2(l)
predict whether the reaction above would have an increase or a decrease in entropy.

decrease in entropy

Explanation:

Entropy is the measure of randomness or disorder of a system. If a system moves from  an ordered arrangement to a disordered arrangement, the entropy is said to decrease and vice versa.

In the given reaction:  

The gaseous oxygen is converting to solid oxygen, thus the randomness is decreasing as the molecules are getting closer and the intermolecular forces are getting stronger. Thus the entropy also decreases.

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i think it's letter a.