List Yes/No

A dropdown with two fixed options. The value is a boolean, exactly as with Checkbox — what differs is how the question reads in the form.

Settings: default, width, required.

Output:

{if $in_stock}
    <span class="in-stock">In stock</span>
{else}
    <span class="out-of-stock">Out of stock</span>
{/if}

When to prefer it over a checkbox

A checkbox reads as an option the editor may leave alone. A dropdown reads as a question that expects an answer — and an unanswered one is visible in the form, which an unticked box never is.