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

$[$ opcode $|$ rd $|$ rs $|$ 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 $6$ bits long. What would be the largest number of unique instruction opcodes that this instruction format would allows to encode?

$8$

$3$

Answer Not Listed

$32$

$64$

$16$