Directions: Select the most appropriate option to substitute the underlined segment in the given sentence.

The company has opened twenty regional offices of whom only eight are currently functional.

The correct option to substitute the underlined segment is of which only eight are currently functional.

Explanation:
1. Use of Relative Pronoun ('which' vs. 'whom'): The phrase refers back to "twenty regional offices", which are inanimate things/objects. The relative pronoun 'whom' is used exclusively for people, whereas 'which' is used for non-human entities, objects, or things. Therefore, "of whom" is incorrect and must be replaced with "of which".
2. Subject-Verb Agreement: The phrase "only eight" refers to a plural quantity (eight offices). Thus, it requires a plural verb ('are') rather than a singular verb ('is').
3. Tense and Usage: Present tense verb 'are' correctly aligns with the adverb "currently".

Breakdown of Options:
• of which only eight is currently functional: Incorrect because it uses the singular verb 'is' for the plural subject 'eight'.
• of whom only eight are currently functional: Incorrect because 'whom' cannot be used for inanimate objects like regional offices.
• of which only eight are currently functional: Correct because it properly uses 'which' for objects and 'are' for plural subject agreement.
• of whom only eight have been currently functional: Incorrect because of the wrong pronoun 'whom' and redundant tense usage with 'currently'.