This question tests your understanding of verb forms and gerunds/infinitives in English grammar. Let's analyze the sentence structure step by step:

Step 1: Identify the first blank
The phrase "a rough way of _____" requires a gerund (verb + -ing) because "of" is a preposition and must be followed by a noun or gerund. "Comparing" is the correct gerund form.

Step 2: Identify the second blank
The structure is "ability _____ something." The correct pattern is "ability to do something," requiring the infinitive "to produce."

Step 3: Identify the third blank
The phrase "_____ into account" is a participle phrase modifying the main clause. Since it's an active action (the GNP calculation itself takes population into account), we need the present participle "taking."

Final Answer: The correct option is "comparing / to produce / taking"

Related Grammar Concepts:

Gerunds: Verb forms ending in -ing that function as nouns. They are used after prepositions like "of," "for," "about."

Infinitives: The "to + verb" form used to express purpose or after certain nouns like "ability," "way," "opportunity."

Participle Phrases: Verb forms used as adjectives. Present participles (-ing) show active meaning, while past participles (-ed) show passive meaning.

Key Formula:
Preposition + Gerund = of comparing, for producing, about taking
Noun + Infinitive = ability to produce, way to compare, need to take