Four sentences are given below as A, B, C and D. Choose the sentence that is grammatically incorrect in the use of the phrasal verb boil down to in the context of a war of succession.

Introduction / Context:
This question tests your understanding of English phrasal verbs, specifically the expression boil down to, which is widely used in political and historical writing to express that a complex conflict is essentially about one core factor. Mastering such fixed expressions is essential for exams that assess advanced grammar and usage, especially in error spotting and sentence improvement sections.

Given Data / Assumptions:
The focus is on four sentences about a war of succession.
Each sentence uses a variation of the verb boil with different particles such as down, on, and to.
You must identify which sentence is grammatically incorrect in standard written English.
The context is a struggle for power during a war of succession.
Assume modern standard English and formal register are required.

Concept / Approach:
In English, many verbs combine with particles to form phrasal verbs that have fixed patterns. The expression boil down to means that something can be reduced to a basic explanation or cause. It always takes the preposition to followed by a noun phrase or clause. Changing the particle, as in boil down on, or breaking the pattern usually makes the expression ungrammatical or unnatural. The approach is to check which sentence violates this fixed usage.

Step-by-Step Solution:
Step 1: Recall that the correct phrasal verb is boil down to, meaning to be mainly about something. Step 2: Examine option A. It says boiled down to who had more power, which correctly follows the pattern verb + down + to + clause. Step 3: Examine option B. It says boiled down to the question of who held more power, which is also correct and even slightly more explicit and formal. Step 4: Examine option D. It says boiled down to which faction had greater power, which again follows the standard pattern and is acceptable. Step 5: Examine option C. It says boiled down on who had more power. Here the particle on replaces to, which breaks the fixed pattern of the phrasal verb and makes the sentence incorrect. Step 6: Conclude that only option C misuses the phrasal verb and is therefore the grammatically incorrect sentence.

Verification / Alternative check:
A quick way to verify is to replace the entire clause after the phrasal verb with a simple noun phrase. For example, the conflict boiled down to power is correct. However, the conflict boiled down on power sounds wrong to a native speaker and does not appear in standard references. This confirms that only to can follow boiled down in this meaning. You can also compare with similar verbs like come down to or reduce to, which all take to, not on, reinforcing the pattern.

Why Other Options Are Wrong:
Option A is acceptable because it uses boiled down to with a clause who had more power, which is a natural structure.
Option B is acceptable because it adds the phrase the question of but retains boiled down to and remains grammatically sound.
Option D is acceptable because it uses boiled down to followed by which faction had greater power, which is also correct usage.
Only option C replaces to with on, so it is the incorrect sentence we need to select.

Common Pitfalls:
A common mistake in phrasal verb questions is to focus only on overall meaning and ignore small function words like prepositions. Many learners think that on, to, and about are interchangeable, but in fixed expressions they are not. Another pitfall is to assume that if the sentence seems understandable, it must be correct, but examinations usually require idiomatic standard usage, not just approximate meaning.

Final Answer:
The grammatically incorrect sentence is Option C. The war of succession ultimately boiled down on who had more power.