High Frequency Traders are

Traders of broadcast frequency used for electronic communications equipment

Illegal in U. S. secondary markets for securities

Firms that use high-speed computers and communications networks to execute millisecond trades for themselves

Firms that use fiber optic cable to route orders to exchanges for customer trades

Firms that use high-speed computers and communications networks to execute millisecond trades for themselves

Explanation:

High Frequency traders are different entities that take part in the market that use powerful computers to be able to be fast at answering to news and have a big number of trades. These traders use advance tools to evaluate the markets and anticipate trends quickly to obtain profits from the trades they make. According to this, the answer is that High Frequency Traders are Firms that use high-speed computers and communications networks to execute millisecond trades for themselves.