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Which of the following is true

  1. Identity element is unique for a group
  2. Inverse is unique for each element of a group
  3. If $a, b$ group $\text{G}$ then $(a b)^{-1} = a^{-1} b^{-1}$
  4. for monoid if $ax = ay$ then $x = y.$
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Options A, B are the properties of the group

C.  $(ab) \neq a^{-1} b^{-1}$ for $(ab)^{-1} = b^{-1} a^{-1}$
$\qquad \quad = (ab).b^{-1}.a^{-1} = e$
$\qquad \quad = a(bb^{-1})a^{-1}$
$\qquad  \quad = aea^{-1}$
$\qquad  \quad  = aa^{-1}$
$\qquad  \quad  = e$
Thus option C is wrong.

D. For monoid inverse does not exist.

$\therefore$ Therefore left cancellation property not possible.
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