Serial format means digital signals are:

ANS:B - sent over one conductor sequentially.

In serial format the data is sent through a single line and data is transferred from one source to another source one by one, 1 bit at a time or sequentially.

In the parallel format, the data is transmitted through multiple lines.
EXAMPLE:

8-bit data transfer =
In parallel all 8 bit transferred at a time by 8 transmission line so it takes 1 CLK pulse for data transmission.

In serial 8 bit transferred one by one ie 1 bit at a time so it takes 8 CLK pulses for data transmission.