Create a Set of Radio Buttons

Summary

Radio buttons are buttons that only allowed one button to be selected at any given time.
Radio buttons are a type of input.
Each of your radio buttons can be nested within its own label element. By wrapping an input element inside of a label element it will automatically associate the radio button input with the label element surrounding it.
All related radio buttons should have the same name attribute to create a radio button group.
Should include a for attribute on the label element with a value that matches the value of the id attribute of the input element.

Final Code

<h2>CatPhotoApp</h2>
<main>
  <p>Click here to view more <a href="#">cat photos</a>.</p>

  <a href="#"><img src="https://www.bit.ly/fcc-relaxing-cat" alt="A cute orange..."></a>

  <p>Things cats love:</p>
  <ul>
    <li>cat nip</li>
    <li>laser pointers</li>
    <li>lasagna</li>
  </ul>

  <p>Top 3 things cats hate:</p>
  <ol>
    <li>flea treatment</li>
    <li>thunder</li>
    <li>other cats</li>
  </ol>

  <form action="https://www.freecatphotoapp.com/submit-cat-photo">
    <input type="text" placeholder="cat photo URL" required>
    <button type="submit">Submit</button>
    <label for="indoor"><input id="indoor" type="radio" name="indoor-outdoor"> Indoor</label>
    <label for="outdoor"><input id="outdoor" type="radio" name="indoor-outdoor"> Outdoor</label><br>
    <input type="text" placeholder="cat photo URL" required>
    <button type="submit">Submit</button>
  </form>

</main>