A good answer might be:

Yes. If the characters to be compared are loaded in the low-order byte of two registers, and the rest of the bits are zero, either instruction could be used.

Set on Less Than Immediate

The other two set instructions compare an operand register with an immediate value in the instruction. There is a version for two's complement: 

                   #  $s and imm contain   
                   #  two's comp. integers
                   #
slti  d,s,imm      #  if ( $s < imm )
                   #    d <-- 1
                   #  else
                   #    d <-- 0

And a version for unsigned integers: 

                   #  $s and imm contain   
                   #  unsigned integers
                   #
sltiu  d,s,imm     #  if ( $s < imm )
                   #    d <-- 1
                   #  else
                   #    d <-- 0

In both, the immediate field of the machine instruction is 16 bits wide.

QUESTION 4:

How is the 16-bit immediate field extended to 32 bits for the comparison? By sign extention or by zero extension?