a) <form>
b) <optgroup>
c) <output>
d) <option>

Answer: b
Explanation: HTML form element is used in its <form> tag having its own structure which is used by users for various tasks. If you have a long list of options, groups of related options are easier to handle for a user. Result of calculation (script) is represented by output tag. A drop down list of items is defined by <option> tag in HTML.