[ Answer ]
One of the softball players left their bat at the field after practice.
[ Explanation ]
Their - Belonging to them
They're - Replaces the word(s) "They are"
There - A position
Out of the options Their, They're and There, the option Their is the correct option. This type of their means that the item belongs to them. The other two types are incorrect and out of context for this sentence.
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