Select the correct option for most meta directing group:
a) NO$_{2}$
b) OCH$_{3}$
c) CHO
d) CONH$_{2}$

Hint: To understand the meta directing groups you should have a better knowledge about the electron withdrawing groups (EWGs) and electron donating groups (EDGs), so that you can easily identify the groups.

Complete answer:
Before proceeding to the question, you should know about some of the terms:
-Directing groups: In an electrophilic aromatic substitution, a benzene ring substituent which influences the benzene ring nucleophilicity and arenium ion stability. They also control where the new substituent groups appear in the product.
-Electron withdrawing groups: electron withdrawing groups are generally meta directors for the electrophilic aromatic substitution. It decreases the electron density in a molecule through carbon atoms it is bonded to by reducing electron density on adjacent carbon atoms and making them electron deficient.
-We can identify electron withdrawing groups by: groups which contain pi bonds to electronegative atoms. For example: Nitro groups(-NO$_{2}$), ketones(-C=OR), cyano groups(-CN), carboxylic acid(-COOH) etc.
-Electron donating groups: electron donating groups are generally ortho/para directors for the electrophilic aromatic substitution. It increases the electron density in a molecule through carbon atoms it is bonded to by increasing electron density on adjacent carbon atoms and making them electron rich compounds.
-We can identify electron donating groups by: groups containing lone pairs to donate. For example: (-O$^{-}$), alcohol groups(-OH$^{-}$), amine groups (-NH$_{2}$ or -NR$_{2}$), ethers(-OR) etc.
-From above we can see that nitro (-NO$_{2}$) groups are electron withdrawing groups and therefore, they are meta directing.
Hence option (A) is the correct answer.

Note:
You should only need to identify the (EWGs and EDGs) and if possible, try to remember the above examples which will help you a lot in future.