How to use column name as a variable?

Question

I have a dataframe as below.

d={
    \'id\' :[1,2,3,4,5],
    \'X1\':[25,12,3,7,1],
    \'X2\':[3,45,3,1,6],
    \'X3\':[6,23,9,3,2]
}

df=pd.DataFrame(d, col