"She is riding a bicycle" 是表示她正在骑自行车的正确英文表达方式。
其中，"ride" 是动词，意思是"骑"，而 "a bicycle" 是名词短语，意思是"一辆自行车"。
例句：
She is riding her bicycle to work every day.
(她每天骑自行车去上班)
The pttle girl is riding her first bicycle.
(小女孩正在骑她的第一辆自行车)
He enjoys riding his bicycle through the countryside on weekends.
(他喜欢在周末骑自行车穿过乡村)
来源: ride is a verb which came from Old Engpsh rīdan "to ride" and bicycle is a noun comes from 19th century, combination of "bi-" (two) and "cycle" (wheel).