Explanation:

This question asks for the correct meaning of the word 'Ubiquitous'. 'Ubiquitous' is an adjective that describes something that is present, appearing, or found everywhere. \nOption A: 'extremely rare' is the antonym of 'ubiquitous'. If something is ubiquitous, it is common and widespread, not rare. \nOption B: 'difficult to understand' describes something complex or obscure. This has no relation to the meaning of 'ubiquitous'. \nOption C: 'being present everywhere at once' perfectly matches the definition of 'ubiquitous'. For example, 'smartphones are ubiquitous in modern society' means they are found everywhere. \nOption D: 'not willing to continue' describes a lack of perseverance or desire to proceed. This is unrelated to the meaning of 'ubiquitous'. \nTherefore, the correct meaning of 'Ubiquitous' is 'being present everywhere at once'.