Which of the following is a correct way to rename the 'old_column' to 'new_column' in a Pandas DataFrame 'df?
a. df.columns = ['new_column' if col ='old_column' else col for col in df.columns]
b. df.columns.rename('old_column', 'new_column')
c. df.rename('old_column', 'new_column', axis=1)
d. df.rename(columns=\{'old_column': 'new_column'\})