Select the most appropriate option that can substitute the underlined segment in the given sentence. If there is no need to substitute it, select ‘No substitution’.
The woman to who I sold my house was a criminal.

The correct answer is "to whom I sold".

Let's break down the grammatical rules to understand why this substitution is necessary:
1. Who vs. Whom: The pronoun "who" is a subjective pronoun, meaning it functions as the subject of a verb (e.g., "Who called you?"). The pronoun "whom" is an objective pronoun, meaning it functions as the object of a verb or a preposition (e.g., "To whom did you speak?"). In the given sentence, the pronoun is the object of the preposition "to". Therefore, we must use "whom" instead of "who".
2. Tense Consistency: The original sentence refers to a completed action in the past ("was a criminal"). Therefore, the action of selling the house must also be in the past tense ("sold") rather than the present tense ("sell"). This rules out "to whom I sell".
3. Sentence Structure: The relative clause needs a subject ("I") to perform the action of selling. The option "to whom sold" lacks a subject, making it grammatically incomplete.

By combining these rules, the underlined segment "to who I sold" must be substituted with "to whom I sold" to make the sentence grammatically correct: "The woman to whom I sold my house was a criminal."