Which of the following observations posed a challenge to Mendeleev’s periodic law?

(a) Position of alkali metals (b) Position of isotopes (c) Anomalies of atomic weights and properties of elements (d) Placing of dissimilar elements in the same group

Mendeleev, in his periodic table, arranged the elements according to their increasing atomic masses but gave priority to the similarity in the fomulas of the oxides and hydrides while groupig them. As a result at some places, an element with a greater atomic mass was placed before an element with a lower atomic mass. For example, Co (atomic mass = 58.93 u) was placed before Ni (atomic mass = 58.71 u), which challenged the law that Mendleeev gave, that the properties of elements are periodic funtions of their atomic masses. Moreover the isotopes of an element have different atomic masses, but could not be alloted separate places in the periodic table on account of the similarity in the formulas their oxides and hydrides.  Therefore, (c) and (d) posed a challenge to Mendeleev's periodic law.