What is the main source for the study of taxonomy?

Statement $S$: Rani Jijamata Udyan is located in Mumbai.
Reason $R$: Himalayan Zoological Park is located in Gangtok.
$(a)$ Both $S$ and $R$ are true,and $R$ is the correct explanation of $S$.
$(b)$ Both $S$ and $R$ are true,but $R$ is not the correct explanation of $S$.
$(c)$ $S$ is true and $R$ is false.
$(d)$ $S$ is false and $R$ is true.

$S$ Statement: Herbaria are generally associated with botanical gardens and educational or research institutions.
$R$ Reason: $A$ botanical garden is a planned collection of trees,shrubs,herbs,climbers,and other living plants brought from different parts of the world for scientific purposes.
$(a)$ Both $S$ and $R$ are true,and $R$ is the correct explanation of $S$.
$(b)$ Both $S$ and $R$ are true,but $R$ is not the correct explanation of $S$.
$(c)$ $S$ is true and $R$ is false.
$(d)$ $S$ is false and $R$ is true.

$A$ $Couplet$ represents the choice made between two opposite options,which results in:

Which of the following is a famous botanical garden located in England?

$A$ student of taxonomy was puzzled when told by his professor to look for a key to identify a plant. He went to his friend to clarify what 'Key' the professor was referring to? What would the friend explain to him?