Marie uses a browser to visit a blog. how is the blog uniquely identified?

Since you didn't give any choices, I will do my best to explain.

Assuming that the question is asking about how the blog is addressed over the network, it has a unique 12 digit number called an IPv4 Address. There's another variation which isn't as commonly used which is an IPv6 Address.

When you type in the blogs web address, you're just using a string to route you to the IPv4 Address. Think of the string of letters in your address bar as a mask, that's basically all it is.

answerpixels;