Reactivity of Aromatic Compounds in Electrophilic Substitution

A high school organic chemistry student is studying aromatic compounds and their reactivity patterns. He learns about electrophilic aromatic substitution (EAS) reactions and starts to question how different substituents on a benzene ring influence its reactivity and orientation. Considering a secondary aromatic compound, 1,4-dimethylbenzene (also known as p-xylene), which of the following substituents, when introduced to the aromatic system, would most likely lead to the most rapid rate of electrophilic substitution?