A function can convert hexadecimal inputs into corresponding binary outputs by byte. For example, if we give "BEEF" as the input to the function, the output would be $"1110111110111110"($EFBE in hex$).$ The output is stored in a register $($the size of the register is $64$ bits which is enough to store the binary form of a maximum of $16$ characters of hexadecimal value$).$ The function is built in such a way that the input hexadecimal value is converted to binary byte by byte, i$.$e$.,$ the function first converts the least significant byte $$EF$$ into $11101111$ and then $$BE$$ into $10111110$ and so on$.$ After computing the binary form of $$EF$,$ the function stores the binary form $11101111$ into the least significant bits of the output register $($i$.$e$.,$ the bits $7,6,5,4,3,2,1,0).$ Before storing the result of the next computation, the output register which holds $"$EF$"$ is first shifted left by $8$ places, and the result for the next computation is stored in the least significant bits $($bits $7,6,5,4,3,2,1,0).$ The process repeats till the end of the hexadecimal string.

$($a$)$ Which of the following block types is needed to shift the bits in the output register? $($assume that only one block is used$)$