Which gas is mixed with propane to detect its odour?

Thiols and mercaptans are a CLASS of organic molecules.  They are two different names for the same class

.. R - SH

where R is any organic C-H chain. (alkyl and aryl included)

here's an example

.. CH3-CH2-CH2-CH2-SH

n-butyl mercaptan.... n-butanel thiol..... butan-1-thio are all names for that 1 compound.

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mercaptans (aka thiols) are generally very pungent.  n-butyl mercaptan smells like skunks.  And mercaptans can commonly by detected by human noses is very small concentrations.  less than 1 part per billion.. ppb

t-butyl mercaptan is 1 chemical used as odorants in natural gas and propane.

.. .. . ..CH3

. . .. . ..|

CH3 - C - SH

.. .. .. . |

. .. .. .CH3

it has an odor threshold of 0.3 ppb and you know the smell

another is ethyl mercaptan

.. CH3-CH2-SH

and another is methyl mercaptan aka methane thiol

.. CH3-SH

and another

. CH3-S-CH3.. .. . .dimethylsulfide

and another

.. CH3-CH2-CH2-SH... . n-propyl mercaptan

.. CH3-CH(CH3)-SH.. . ..isopropyl mercaptan

etc

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often odorants are manufactured by a chemical company and are specific blends of those chemicals + others depending on the application and the gas they are used to odorize.  For example

.. Scentinal A (by Chevron) is 99% ethyl mercaptan.

.. Scentinal N is 75% t-butyl and 25% isopropyl mercaptan

.. Scentinal F-20 is 80% t-butyl and 20% dimethyl sulfide (CH3)2S

.. Scentinal T is thiophane (cyclic C4H8S)

.. Scentinal P is n-propyl, isopropyl and n-butyl mercaptans

.. Scentinal O a blend of 6 different sulfides and mercaptans.

.. Scentinal H  hexane + t-butyl mercaptane (20%)

.. Scentinal Eis n-propyl, isopropyl and n-butyl mercaptans

etc.

All of which COULD be used to odorize propane.  Which one is used depends on (1) local standards, (2) local availability, (3) cost, the propane supplier, etc.  All of them are compounds containing sulfur.

wrong question. What you meant to ask is: which gas is added to fuelgases so humans can smell a leak?

answer: https://en.wikipedia.org/wiki/Methanethiol

Mercaptan.  Smells like sulphur.