Question
Consider the binary operation $*$ and $o$ defined by the following tables on set $S = \{a, b, c, d\}.$
$o$ $a$ $b$ $c$ $d$
$a$ $a$ $a$ $a$ $a$
$b$ $a$ $b$ $c$ $d$
$c$ $a$ $c$ $d$ $b$
$d$ $a$ $d$ $b$ $c$

Answer

Commutativity:The table is symmetrical about the leading element.
It means that $o$ is commutative on $S$.
$a o (b o c) = a o c$
$= a$
$(a o b) o c = a o c$
$= a$
thus,
$a o (b o c) = (a o b) o$ 
$\text{c }\forall\text{ a, b, c}\in\text{S}$
Associativity:
Therefore$, o$ is associative on $S.$
Finding identity element:
We observe that the second row of the composition table coincides with the top$-$most row and the first column coincides with the left$-$most column.
These two intersect at $b.$
Implies that $x o b = b o x$
$=\text{x, }\forall\text{ x}\in\text{S}$
Therefore,
$b$ is the identity element.
Finding inverse elements:
In the first row, we don't have $b,$
i.e. there does not exist an element $x$ such that $a o x = x o a = b.$
Therefore,
$a^{-1}$ does not exists.
$b o b = b$
Implies that $b^{-1 }= b$
$c o d = b$
Implies that $c^{-1} = d$
$d o c = b$
Implies that $d^{-1} = c$

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