The correct answer is **普及 (ふきゅう)**. This word means "diffusion," "spread," or "popularization." It perfectly describes the process of something, like a technology or an idea, becoming widely used and common among the general public. In the sentence, 'The (spread/diffusion) of the internet has dramatically changed our lives,' **普及** is the most suitable word. It's the widespread adoption and integration of the internet into daily life that caused the dramatic change, and **普及** captures this nuance precisely. Common Mistakes / Nuances: ❌ : **発展 (はってん)** means "development" or "growth." While the internet has certainly developed, this word focuses on the technological advancement and improvement of the thing itself, not its adoption by people. For example, you would talk about 「技術の発展」 (technological development) or 「経済の発展」 (economic development). The sentence emphasizes the change in *our lives*, which is a direct result of widespread *use* (普及), not just the improvement of the technology (発展). ❌ : **拡大 (かくだい)** means "expansion," "enlargement," or "magnification." It refers to an increase in physical size, scale, or scope. For example, one might say 「事業の拡大」 (business expansion). While the internet network has physically expanded, **拡大** doesn't convey the social nuance of it becoming a common tool used by ordinary people. **普及** is about social penetration, whereas **拡大** is about an increase in scale. ❌ : **分布 (ぶんぷ)** means "distribution." It describes how something is spread out over a geographical area or within a population, often in a static, statistical sense. For instance, a map might show the 「人口の分布」 (population distribution). It describes the *state* of how something is distributed, not the dynamic *process* of it spreading and becoming popular. The sentence implies a process over time that changed lives, which makes the dynamic meaning of **普及** more appropriate.

正解は **普及（ふきゅう）** です。この言葉は、ある物事（技術、思想、習慣など）が広く一般に行き渡り、多くの人々に使われるようになることを意味します。「インターネットの（普及）は、私たちの生活を劇的に変えた」という文では、インターネットが世の中に広まって生活の一部になったことが、生活を劇的に変えた原因です。このニュアンスを的確に表現するのが **普及** です。 Common Mistakes / Nuances: ❌ : **発展 (はってん)** means "development" or "growth." While the internet has certainly developed, this word focuses on the technological advancement and improvement of the thing itself, not its adoption by people. For example, you would talk about 「技術の発展」 (technological development) or 「経済の発展」 (economic development). The sentence emphasizes the change in *our lives*, which is a direct result of widespread *use* (普及), not just the improvement of the technology (発展). ❌ : **拡大 (かくだい)** means "expansion," "enlargement," or "magnification." It refers to an increase in physical size, scale, or scope. For example, one might say 「事業の拡大」 (business expansion). While the internet network has physically expanded, **拡大** doesn't convey the social nuance of it becoming a common tool used by ordinary people. **普及** is about social penetration, whereas **拡大** is about an increase in scale. ❌ : **分布 (ぶんぷ)** means "distribution." It describes how something is spread out over a geographical area or within a population, often in a static, statistical sense. For instance, a map might show the 「人口の分布」 (population distribution). It describes the *state* of how something is distributed, not the dynamic *process* of it spreading and becoming popular. The sentence implies a process over time that changed lives, which makes the dynamic meaning of **普及** more appropriate.