Explanation: A complete sentence must have a subject (the person or thing that performs the action) and a predicate formed by at least a verb (it can also have modifiers and complements). In the given group of words, we can see an example of an incomplete sentence, because "the girl ridin her bike" is a subject ("riding her bike" is a gerund that acts like an adjective modifying the noun "girl"), but it doesn't have a verb that indicates the action.
