Several ______attended the two-day economic summit.

Analyze the question: The sentence needs the correct plural form of the compound noun "head of state." The word "Several" tells us we need a plural.

Concept: In compound nouns made up of a noun + preposition + noun (like head of state, mother-in-law, commander-in-chief, man of war), the main noun (the first one) takes the plural "s". The noun after the preposition stays the same.

Examples:

• attorney-general → attorneys-general
• passer-by → passers-by
• minister of state → ministers of state
• head of state → heads of state

Step-by-step solution:

1. "Several" requires a plural noun, so A (head of state) and D (head of states) are wrong, because "head" is still singular in both.
2. Between B and C: the main noun is "head", so it takes the "s". "State" stays singular because it only describes what kind of head is meant (a head of a state/country).
3. B (heads of states) wrongly pluralizes "state".
4. C (heads of state) is the standard correct form.

Correct sentence: "Several heads of state attended the two-day economic summit."

Answer: C

Tip: For compounds with a preposition (of, in, at), pluralize the first/main word only: heads of state, sons-in-law, commanders-in-chief, men of war.