Thiol group is present in:

We begin by recalling the definition of a thiol group. A thiol group is written as $$-SH$$ and is the sulfur analogue of the alcohol $$-OH$$ group. Any molecule that contains the $$-SH$$ functional group is said to possess a thiol (sulfhydryl) group.

Now we examine each option one by one and look for the presence or absence of the $$-SH$$ group.

Option A - Cysteine: The structure of the amino acid cysteine can be written schematically as

$$\mathrm{HS{-}CH_2{-}CH(NH_2){-}COOH}$$

Here we clearly see the fragment $$\mathrm{-SH}$$ attached to the $$\mathrm{CH_2}$$ carbon. This confirms that cysteine indeed contains a thiol group.

Option B - Methionine: The amino acid methionine has the chain

$$\mathrm{CH_3{-}S{-}CH_2{-}CH_2{-}CH(NH_2){-}COOH}$$

The sulfur atom here is part of a thio-ether linkage $$\mathrm{-S-}$$ between two carbon atoms. Because the sulfur is bonded to two carbons and no hydrogen, there is no $$-SH$$ (thiol) group in methionine.

Option C - Cytosine: Cytosine is a nitrogenous base found in nucleic acids. Its ring contains nitrogen and oxygen atoms but no sulfur at all, so a thiol group is completely absent.

Option D - Cystine: Cystine is formed when two cysteine molecules oxidize to create a disulfide bond:

$$2\ \mathrm{HS{-}CH_2{-}CH(NH_2){-}COOH} \;\;\rightarrow\;\; \mathrm{HOOC{-}CH(NH_2){-}CH_2{-}S{-}S{-}CH_2{-}CH(NH_2){-}COOH}$$

In the product, the two $$\mathrm{-SH}$$ groups are converted into one $$\mathrm{-S{-}S-}$$ bridge (disulfide). Therefore cystine lacks a free thiol group.

After this detailed comparison, only cysteine (Option A) contains the required $$-SH$$ functional group.

Hence, the correct answer is Option A.