Correct option is (a).
Reason:
Let fix notation: "\\\\\\leq" denote subgroup.
Now, we have given "G" is a group,"H\\leq G \\&K\\leq G" .
Claim:
"H\\cap K\\leq G"Proof:
It is sufficient to show "H\\cap K \\leq G" if we show "H\\cap K" closed under multiplication and taking inverse in it.
Let, for every "h,k\\in H\\cap K \\implies h\\in H \\& h\\in K" also "k\\in H \\& k\\in K" ,thus
Hence,
"hk\\in H\\cap K"
Which implies "H\\cap K" is closed under multiplication.
Now, consider
"\\forall h\\in H\\cap K\\implies h\\in H\\&h\\in K"
Since, "H,K\\leq G" ,
"h^{-1}\\in H\\&h^{-1}\\in K\\implies h^{-1}\\in H\\cap K"Thus, "H\\cap K" is closed under taking inverse.
Therefore,
"H\\cap K \\leq G"
For remaining 3 options we observe that if we choose "G=S_3" and "H=\\{e,(12)\\}\\&K=\\{e,(13)\\}" , does not qualify the reaming options to be subgroup of "G" .
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