ANS:A - register

Register is use to store the data bits.

Decoder is use to convert a code into a decoded form. Mainly example 2-4 decoder 3-8 decoder. Mean 2 input 4 output (2^2 n 2^3).

Counter is a sequential circuit example which count the data bits.

Multiplexer is a device which allow transfer of data bits from different source on a single channel. So for multiple input there is single output.