Concept:Ordinal numbers (like "first", "ninth") require the definite article "the" before them.
Explanation:The sentence has three underlined parts: (a) "Once considered ninth planet", (b) "Pluto is today listed as the", (c) "largest dwarf planet of the solar system".
The error is in part (a).
The word "ninth" is an ordinal number.
In English grammar, definite article "the" is used before ordinal numbers and superlative adjectives.
For example: "The 20th century" (ordinal) and "the bloodiest wars" (superlative).
Therefore, "ninth planet" should be written as "the ninth planet".
No errors are present in parts (b) or (c).
Thus, the sentence is incorrect only in part (a).
Answer:(a)