Directions: Choose the appropriate preposition to complete the sentence.

As the eldest child, he was declared the rightful heir ________ the family fortune.

The correct option is to.

Explanation:
In English grammar, certain nouns pair with specific prepositions to express standard legal and familial relationships. The noun heir is conventionally followed by the preposition to when specifying the property, throne, title, or fortune that a person is entitled to receive upon inheritance.

Usage & Rules:
• Heir to: Used when referring to the property, title, or status being inherited (e.g., "heir to the throne", "heir to an immense fortune").
• Completing the sentence gives: "As the eldest child, he was declared the rightful heir to the family fortune."

Why other options are incorrect:
• for: The preposition "for" does not form a standard prepositional collocation with "heir" to express inheritance.
• by: "By" indicates agency or method (e.g., "done by"), which does not express the legal relationship of inheritance.
• of: While "heir of" can sometimes be used when referring directly to the person who leaves the estate (e.g., "he the heir of his father"), when designating the actual property or fortune itself, to is the standard preposition.