The correct choice is 増加 (ぞうか). The verb 増加する (ぞうかする) specifically means 'to increase' in number or quantity. In a sentence like 「スマートフォンの利用者が増加している」(The number of smartphone users is increasing), it perfectly captures the idea of a numerical count growing larger over time. Common Mistakes / Nuances: ❌ : 追加 (ついか) means 'addition' or 'supplement'. It is used when you actively add a new item or element to an existing set. For example, 『メニューに新しいデザートを追加する』(add a new dessert to the menu) or 『リストに名前を追加する』(add a name to the list). It doesn't fit the context of a population or user base growing naturally or as a trend; it implies a discrete act of adding something. ❌ : 拡大 (かくだい) means 'expansion', 'enlargement', or 'magnification'. It refers to an increase in physical size, area, scope, or scale. For instance, you would use it for 『事業を拡大する』(to expand a business) or 『地図を拡大する』(to enlarge a map). While the smartphone *market* could be said to be expanding, when talking specifically about the increase in the *number* of users, 増加 is the more precise and natural term. ❌ : 発展 (はってん) means 'development', 'growth', or 'progress'. Crucially, it implies a qualitative improvement or advancement, not just a quantitative increase. It describes something becoming more advanced, complex, or mature. For example, 『経済が発展する』(the economy develops) or 『技術が発展する』(technology advances). It is not used for a simple numerical increase, which is what is being described in the context of user numbers.

正解は「増加（ぞうか）」です。動詞の「増加する」は、数や量が増えることを具体的に指します。例えば、「スマートフォンの利用者が増加している」という文では、利用者の「数」が時間と共に多くなっていく状況を的確に表現しています。 Common Mistakes / Nuances: ❌ : 追加 (ついか) means 'addition' or 'supplement'. It is used when you actively add a new item or element to an existing set. For example, 『メニューに新しいデザートを追加する』(add a new dessert to the menu) or 『リストに名前を追加する』(add a name to the list). It doesn't fit the context of a population or user base growing naturally or as a trend; it implies a discrete act of adding something. ❌ : 拡大 (かくだい) means 'expansion', 'enlargement', or 'magnification'. It refers to an increase in physical size, area, scope, or scale. For instance, you would use it for 『事業を拡大する』(to expand a business) or 『地図を拡大する』(to enlarge a map). While the smartphone *market* could be said to be expanding, when talking specifically about the increase in the *number* of users, 増加 is the more precise and natural term. ❌ : 発展 (はってん) means 'development', 'growth', or 'progress'. Crucially, it implies a qualitative improvement or advancement, not just a quantitative increase. It describes something becoming more advanced, complex, or mature. For example, 『経済が発展する』(the economy develops) or 『技術が発展する』(technology advances). It is not used for a simple numerical increase, which is what is being described in the context of user numbers.