Suppose a datapath has three operand busses, $128$ instruction types, and $64$ registers where each register is $32$ bits wide. Immediate operands can be in the range of $+-64K.$ Design an instruction format for instructions that have one operation, one destination register, one source register, and an immediate value. Determine the following values for the resulting instruction format.

Minimum number of bits needed to specify an opcode is

a$)$

Five

b$)$

Six

c$)$

Seven

d$)$

Eight

e$)$

None