Mischmetal is an alloy consisting mainly of:

We begin by recalling the definition of the alloy named “mischmetal.” Mischmetal (from the German word for “mixed metal”) is prepared during the electrolytic reduction of molten lanthanide chlorides obtained from monazite sand. Because the reduction vessel contains only the chlorides of the lanthanoid elements, the metal obtained on electrolysis must necessarily consist of the same lanthanoid elements in their metallic form.

Typical analytical data show that mischmetal contains approximately $$45\%$$ cerium, $$25\%$$ lanthanum, $$17\%$$ neodymium, with smaller amounts of praseodymium and samarium. These percentages together account for practically the whole composition, and every constituent listed belongs to the lanthanoid (4f-series) family of the periodic table.

Now we compare this factual composition with the four options given in the question:

Option A mentions only lanthanoid metals. This matches the list just discussed (Ce, La, Nd, etc.).

Option B refers to “actinoid and transition metals,” but mischmetal is known to contain neither actinoid nor transition metals in any significant amount.

Option C suggests “lanthanoid and actinoid metals.” Again, no actinoid element (Th, U, Pu, etc.) is present in mischmetal.

Option D speaks solely of “actinoid metals,” which clearly contradicts the observed composition.

Consequently, the only statement that correctly describes mischmetal is the one that limits its composition mainly to the lanthanoid metals.

Hence, the correct answer is Option A.