When should I use fields that are disabled, read-only, or hidden?

  

[...] in this case fixed fields are never editable [...]

That is, they are not edit boxes ( labels), but labels labels. So your answer is: Use the C option.

Justifications

I do not know of any rule that says you have to do X or Y. Nor would it make sense to have a rule that dictates how to do it. This is the developer's free choice noting the usability of the system.

Using the readonly feature is just a convenient way where you do not need to create a conditional to write as a label.

Example with PHP,

if ($readonly) {
    echo 'foo';
} else {
    echo '<input type="text" value="foo">';
}

Or, depending on the case you may still need the field in a hidden way:

if ($readonly) {
    echo '<input type="hidden" value="foo">
    foo';
} else {
    echo '<input type="text" value="foo">';
}

If you choose the "readonly" attribute, it could be something like this:

echo '<input type="text" value="foo"'.($readonly? ' readonly': '').'>';

Of course, the performance gain is ridiculous, but it does save a little more processing memory anyway. In addition to having the code cleaner.

I could also play processing to the client:

<input type="text" value="foo" id="foo">
<script>
var readonly = <?php echo $readonly;?>;
$("#foo").prop("readonly", readonly);
</script>

In this third example, the process usage on the server is much smaller compared to the previous two.

In addition, if you want to avoid the situation exposed by @WilliamNovak

  

From your own experience, if you use a disabled field, hide it and   places text in the same format as the label. I've heard a lot of user   speaking "the system does not work, you can not edit the [...] field".

The comment is correct because if misused and depending on the target audience, it can be inconvenient and a better option is to avoid using a simple readonly . If you use, it is recommended to format visually with CSS so it does not give the impression that it is an editable field.

You could solve by applying a formatting in a way that presents the field as disabled. Usually with a more opaque color or by eliminating the borders and background color of input text .

<input type="text" value="foo" id="foo" class="readonly_<?php echo $readonly;?>">

In a CSS leave already pre-defined

.readonly_0 {
    border:1;
    background-color:#05bbcc;
}
.readonly_1 {
    border:0;
    background-color:#fff; /*na mesma cor do fundo da página*/
}

We could use the "disabled" attribute which makes the element opaque, but you may still have the problem described by William.

There are N ways to do these examples above. I stress that they are examples with didactic purpose.

Finally, there is no general rule. Each one defines what is best for your project. As it is something visual, the biggest concern would be with the type of user. Technical users would not cause inconvenience then a simple readonly and making the opaque element would already solve but when dealing with lay (non-technical) users may have situations that waste time with support and customer service.

The main point is to create an intuitive design. Think about UX (user experience). An efficient intuitive design is one that no user has difficulty understanding and does not even need an instruction manual. Of course, it's even more convenient when you can have an efficient UI at a low cost.