Answer:
Capital
Step-by-step explanation:
The meaning of capital and capitol according to the context of the sentence (He invested all of his (capital, capitol) in an internet start-up firm.) is;
Capital: Capital means wealth in the form of money according to Oxford Dictionary.
Capitol: Capitol refers to a building where the legislative body of a government resides, where the functions of the state government are carried out.
Based on the sentence, a person can only invest wealth in the form of money in an internship start-up firm. This makes the answer Capital.