Assume an arbitrary $16-$bit instruction format has the following fields:

Iopcode $|$ rd $|$ is $|$ immediate $]$

rd specifies the destination register, rs a source operand register. A total of $6$ general purpose registers can be used and the immediate field is $5$ bits long. What would be the largest number of unique instruction opcodes that this instruction format would allows to encode?

r

$32$

$8$

Answer Not Listed

$3$

$64$