Later Sachs added a fifth category, electrophones, such as theremins, which produce sound by electronic means. Modern synthesizers and electronic instruments fall in this category. Within each category are many subgroups. The system has been criticised and revised over the years, but remains widely used by ethnomusicologists and organologists.

Answer this question based on the passage: what category of instruments do synthesizers fall in? OK. Let's think. My stream of consciousness:
The relevant information is: Modern synthesizers and electronic instruments fall in this category. The answer is electrophones.