Explanation:

When handling client-side AJAX requests in Salesforce B2C Commerce SFRA, the best practice is to use:
res.json() — to send structured data (JSON) back to the browser for processing by client-side JavaScript.

📦 Why res.json() is best for AJAX:
It returns a JSON-formatted HTTP response, which is easy to handle in JavaScript.
It's ideal for partial page updates — such as cart updates, product info refresh, or stock availability.
It avoids re-rendering a full page, supporting a modern interactive UX.
✅ Example:
server.get('UpdateStock', function (req, res, next) {
var productInfo = {
stock: 12,
productId: '12345'
};
res.json(productInfo);
next();
});

🧠 In the browser:
fetch('/YourController-UpdateStock')
.then(res => res.json())
.then(data => {
document.querySelector('#stockCount').innerText = data.stock;
});

❌ Why the other options are incorrect:

res.render()
Renders an ISML template (used for full pages or HTML fragments); not ideal for AJAX unless you're injecting raw HTML.
res.print()
Sends a raw string response; works, but lacks structure and makes parsing harder for the client. Not modern or clean.

📘 Reference:
SFRA Controller Response Methods
MDN – Fetch API