VOCABULARY — Choose the best synonym (same meaning) of the highlighted word, based on the sentence. Sentence: “Instead of putting up a united front against one common enemy, the medieval states frittered away their energy in INTERNECINE warfare.”

Introduction / Context:
“Internecine” is a historical and political term commonly used to describe conflict within a group, faction, or nation that results in damage on both sides. In essence, it is conflict that destroys participants on the same side or within the same system.

Given Data / Assumptions:

• The context mentions medieval states fighting among themselves rather than uniting.
• We need a synonym that captures intra-group destruction.
• Several options are general negatives; only one specifies mutual internal destruction.

Concept / Approach:
“Internecine” most precisely equals “mutually destructive.” While “baneful,” “pernicious,” and “detrimental” all indicate harm, they do not inherently convey harm to both or all sides within the same group. The defining feature of internecine strife is reciprocal damage among kin, allies, or members of the same polity.

Step-by-Step Solution:
1) Identify the key idea: conflict within related states causing mutual damage.2) Compare options: only “mutually destructive” explicitly encodes reciprocity of harm.3) Select “mutually destructive.”4) Substitute in the sentence to validate meaning.

Verification / Alternative check:
Historical accounts of “internecine wars” often stress brother-against-brother or ally-against-ally contexts, confirming reciprocity is central.

Why Other Options Are Wrong:
B) “baneful” — harmful but not necessarily reciprocal.C) “pernicious” — highly harmful, lacks the mutual aspect.D) “detrimental” — general harm only.

Common Pitfalls:
Choosing a generally negative adjective and missing the key nuance of “mutual” internal destruction.

Final Answer:
mutually destructive