Answer: The correct answer is CAPITAL: He invested all of his capital in an internet start-up firm.
Step-by-step explanation: The word 'capital' makes reference to a large amount of money that is mainly used to set up a business. In that way, 'capital' is closely connected to the idea of investing money on a particular project, such as an internet start-up firm. For that reason, the word that should be part of the sentence is 'capital' and not 'capitol' since 'capitol' refers to a large building used for legislative sessions and; therefore, it is not related to the meaning of the other elements in the sentence.