You need to ensure that the value of an input element is a valid 10-digit...

< input id= " phone "

pattern= " [0-9]{10} "

placeholder= " 0000000000 " >

The correct first attribute is pattern because the requirement is to validate the entered value against a specific format: exactly ten numeric digits and no symbols. The regular expression [0-9]{10} means that only digits from 0 through 9 are allowed, and exactly ten of them must be entered. This prevents values such as 123-456-7890, (123)4567890, or 123 456 7890 because those contain symbols or spaces. The correct second attribute is placeholder, not value, because the input should initially display all zeroes but that displayed text must not be submitted or stored with the form. A placeholder is hint text shown when the field is empty; it disappears when the user enters real data and is not submitted as the control’s value. By contrast, value= " 0000000000 " would actually prepopulate the input and could be submitted if the user did not change it. The required option is not selected because the question focuses on format validation and display hint behavior, not mandatory completion. References/topics: HTML5 form validation, pattern, regular expressions, placeholder, input constraint validation.