Write the IUPAC name of  $\mathrm{C}{\mathrm{H}}_3-(\mathrm{C}{\mathrm{H}}_2{)}_5-\mathrm{CHO}$.
 

An organic compound with a CHO group is an aldehyde. $\mathrm{C}{\mathrm{H}}_3-(\mathrm{C}{\mathrm{H}}_2{)}_5-\mathrm{CHO}$ is an open chain aliphatic aldehyde, which is the given compound. The steps for giving IUPAC names of compounds are as follows.

i) Pick the carbon compound with the longest chain.
${\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_2-\mathrm{CHO}$
There are 7 carbon atoms in the longest chain. Heptane is the parent alkane as a result.
ii) After that, assign numbers to the carbon atoms. Starting with the aldehyde group's carbon, numbers should be assigned, because it is the functional group here so it will be given priority.
iii) Because the carbon chain is free of any further substituents. Therefore, an aldehyde's name is formed by substituting the letter 'al' for the letter 'e'. In this case, Heptane acts as the parent alkane, and the 'e' of heptane is changed to 'al'. Heptanal therefore will be the IUPAC name for the mentioned aldehyde.
Remember that Heptanaldehyde is the common name for heptanal, and in this question, we have been asked to find out the IUPAC name only, so our answer will be Heptanal.

Hence, the correct answer is option A.