Synchronous protocols transmit characters one at a time
Synchronous protocols transmit characters one at a time statement is true.
Bit-oriented protocols are synchronous protocols that view the transmitted data as a stream of bits with no semantics, or meaning. Control codes are defined in terms of bit sequences instead of characters. Synchronization is maintained on an idle line by transmitting a predefined sequence of bits.
The protocols for serial data transfer can be grouped into two types: synchronous and asynchronous. For synchronous data transfer, both the sender and receiver access the data according to the same clock. ... This speed usually does not change after the data transfer starts.
Synchronous serial communication describes a serial communication protocol in which "data is sent in a continuous stream at constant rate." ... For this reason "synchronous communication permits more information to be passed over a circuit per unit time" than asynchronous serial communication.