As per Mendeleev’s prediction, atomic mass of eka-aluminium was:

Correct — C. Mendeleev left gaps in his table for elements not yet discovered and named each after the element standing above the gap, with the Sanskrit prefix eka-, meaning one.

Eka-aluminium was the missing element one place below aluminium, and he put its atomic mass at 68.

It turned up in 1875 as gallium, atomic mass about 69.7. The closeness of that match is what made the periodic law credible → option (c).

Why the others are wrong

• (a)
  72 — 72 was the prediction for eka-silicon, the gap one place below silicon, filled in 1886 by germanium at about 72.6. Right method, wrong gap.
• (b)
  100 — 100 does not belong to this gap at all. The eka-aluminium slot sat just after zinc, whose atomic mass is about 65, so the missing element had to weigh a little more than zinc, not half as much again.
• (d)
  44 — 44 was the prediction for eka-boron, matched by scandium at about 45. Learning the three eka- predictions as a set is what makes these options separable.

Mendeleev arranged the elements by increasing atomic mass and grouped those with similar properties, then took the step that mattered: where no known element fitted, he left the place empty rather than forcing a neighbour into it.

For three of those blanks he predicted properties in detail — atomic mass, density, oxide formula — under the names eka-boron, eka-aluminium and eka-silicon.

Scandium, gallium and germanium were found within fifteen years and matched. That confirmation is why the table survived its own defects, such as the misplacement of isotopes and the reversed pair cobalt and nickel.

Key facts

• Mendeleev predicted eka-aluminium at atomic mass 68 and gallium, discovered in 1875, came in at about 69.7.
• Eka-silicon was predicted at 72 and matched by germanium, discovered in 1886, at about 72.6.
• Eka-boron was predicted at 44 and matched by scandium at about 45.
• Eka- is Sanskrit for one, marking the element one place below a known element in the same group.

Common traps

• Swapping the eka-aluminium and eka-silicon figures, which is exactly why 72 is offered here.
• Quoting the modern atomic mass of gallium instead of the predicted 68.
• Assuming eka- names the element above the gap rather than the one below it.

No directly related past PYQ was found.