React components must start with a capital letter. This is how React distinguishes between DOM elements and custom components.

You must use the setter function from useState (like setCount). Directly modifying state won't trigger a re-render.

When rendering lists, each element needs a unique 'key' prop so React can efficiently track changes.

An empty dependency array tells React to run the effect only once after the initial render (mount).

Use an arrow function to pass arguments. onClick={handleClick(id)} would call the function immediately during render!

The children prop contains whatever is placed between a component's opening and closing tags, allowing component composition.

Both work! Use && for inline rendering in JSX, or if statements in the function body. The ternary without an else (option C) is invalid syntax.

setState schedules an update. React batches these updates for performance and re-renders the component with the new state.