HTML (Hypertext Markup Language) is primarily used for web development. It is a markup language that is used to structure the content of a webpage. HTML provides a standard way to define the elements and layout of a webpage, such as headings, paragraphs, images, links, and tables. It is the backbone of any web page and is essential for creating the structure and layout of a website.

Explanation:

1. HTML:
- HTML stands for Hypertext Markup Language.
- It is the standard markup language for creating web pages.
- HTML tags are used to define the structure and layout of a webpage.
- It is a client-side language, meaning it is processed by the web browser.
- HTML is not a programming language but a markup language.

2. JavaScript:
- JavaScript is a scripting language primarily used for adding interactivity to web pages.
- It is often used alongside HTML and CSS to create dynamic and interactive websites.
- JavaScript can be used for client-side as well as server-side programming.
- It allows for the creation of interactive features such as form validation, image sliders, and dynamic content updates.

3. CSS:
- CSS stands for Cascading Style Sheets.
- It is used to define the visual appearance and layout of HTML elements.
- CSS is used to control the font size, color, spacing, positioning, and other visual aspects of a webpage.
- It works in conjunction with HTML to enhance the visual design of a website.

4. PHP:
- PHP is a server-side scripting language primarily used for web development.
- It is often used in combination with HTML to create dynamic web pages.
- PHP can interact with databases, handle form data, and perform other server-side tasks.
- It is widely used for building web applications and content management systems.

Conclusion:
While all the options mentioned are important in web development, HTML is the primary language used for creating the structure and layout of web pages. JavaScript is used for adding interactivity, CSS is used for styling and layout, and PHP is used for server-side scripting. However, since the question asks for the primary language used in web development, the correct answer is HTML.