React Syntax Issue for Functional Component (Why 4th syntax doesn’t work – Like to know reason) [duplicate]

While creating a functional component I can write this:

A.

function App() { return <h1>Hello</h1> }
export default App; 

B.

export default function App() {return <h1>Hello</h1>}

C.

const App = () => {return <h1>hello</h1>}
export default App; 

D.

export default const App = () => {return <h1>hello</h1>}

While A, B, C — work fine;
Why there is an issue while writing like ‘D’. Any particular reason? Or What’s the issue? Or was it just decision of ‘React Syntax Developers’. Confused, pls help.