Correct:
<form enctype="multipart/form-data" action="index.php" method="post">
Explanation: Answer option C is correct.
In most cases, a user does not need to use any enctype attribute. The reason behind this is that the default value (even enctype is not given in the form) is "application/x-www-form-urlencoded", which is sufficient for almost any kind of form data. However, the default enctype value cannot be used for file uploading. For file uploading, a user must use the "multi-part" HTTP POST method. The syntax of the example form is as follows:
<form enctype="multipart/form-data" action="index.php" method="post">
<input type="hidden" name="MAX_FILE_SIZE" value="50000? />
<input name="filedata" type="file" />
<input type="submit" value="Send file" />
</form>
Answer option D is incorrect. In this form format, form is encoded by certain rules.
Answer option A is incorrect. In this form format, data is sent in the plain text format and any type of encoding is not done.
Answer option B is incorrect. This is not a valid form format.
Reference: https://php.net/manual/en/features.file-upload.php