The correct options are
A It explains why element in the same group have the same chemical properties.
C Isotopes have been placed at same position.
D It is based on classifying elements according to their atomic number.
The modern periodic law is that the properties of the elements are periodic functions of their atomic numbers. The isotopes are not shown separately.
The position of an element in a periodic table depends upon its electronic configuration and properties. Hydrogen resembles the elements of groups I-A, IV-A, and VII-A in some respects. Properties of hydrogen do not completely match any one of the above-mentioned groups. That is why the position of hydrogen is still undecided.