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Elag guestion

The following question is given: Use the Pumping Lemma with length to prove that the following language is nonregular:

$L=\{a{b}^{n}ab,$ with $n>0\}.$

The solution to this question is partly given as follows:

Assume $L=\{\begin{array}{c}a{b}^{n}ab\end{array},$ where $\{$:$n>0\}$ is regular.

Then there exists an FA with, say, $k$ states, that accepts $L.$

Let $w=a{b}^{k}ab$ be a word in $L.$

According to the pumping lemma, $w$ may be written as

$w=xyz$ such that

length $(x)+$ length $(y)k$ AND length $(y)>0$

Which one of the following is not one of the possible correct choices for $y?$

$y=.$

$y$ comprises the first $a-$substring.

$y$ comprises the first $a-$substring followed by at most $(k-1)b\text{'}$s$.$

$y$ comprises at most $(k-1)b\text{'}$s from the first set of $b\text{'}$s$.$