Decoder:
A decoder is a digital circuit that converts coded input signals into a specific set of output signals. It is used in various applications such as data decoding, address decoding, and memory decoding. The main function of a decoder is to select one of the many outputs based on the input code.

Combinational Circuit:
A combinational circuit is a digital circuit that produces an output based on the current input values. It does not have any memory elements, and the output is solely determined by the current input values. Combinational circuits are designed using logic gates such as AND, OR, NOT, etc.

Sequential Circuit:
A sequential circuit is a digital circuit that has memory elements such as flip-flops. The output of a sequential circuit depends not only on the current input values but also on the previous input values and the state of the memory elements. Sequential circuits have a clock signal that controls the timing of their operation.

Logical Circuit:
A logical circuit refers to any circuit that performs logical operations such as AND, OR, NOT, etc. Both combinational and sequential circuits can be considered as logical circuits since they involve logical operations.

Representation for Decoder:
A decoder is a combinational circuit. It takes an n-bit input and produces 2^n outputs. Each output corresponds to a unique input combination. The number of outputs is determined by the number of input lines. For example, a 2-to-4 decoder has 2 input lines and produces 4 output lines.

A decoder can be represented using logic gates such as AND gates and NOT gates. The input lines are connected to the inputs of the AND gates, and their outputs are connected to the output lines. The output lines are activated based on the specific input combination. For example, if the input code is '00', the first output line is activated. If the input code is '01', the second output line is activated, and so on.

Since a decoder does not have any memory elements and its output is solely determined by the input code, it is considered as a combinational circuit. It does not depend on any previous input values or memory states. Therefore, the correct answer is option 'B' - Combinational circuit.