Final answer:
Optional elements are the things in a phrase that are not required, providing additional information or clarification.
Step-by-step explanation:
Optional elements are the things in a phrase that are not required. They are not necessary for the phrase to convey its intended meaning. Optional elements provide additional information or clarify the meaning, but their absence does not affect the grammaticality or correctness of the phrase.
For example, in the phrase 'I went to the store yesterday', the word 'yesterday' is an optional element. It adds information about the time of the action, but the phrase is still grammatically correct and conveys its intended meaning without it.
Therefore, the correct answer is option d) Optional elements.