Esters are relatively unreactive since the leaving group is an alkoxide ion (OR-), which is the conjugate base of an alcohol (a relatively weak acid). 

To answer this question, it's important to recall what sigma bonds and pi bonds are. Sigma bonds are the result of overlapping s orbitals, and are contained in all bonds. Pi bonds, on the other hand, are the result of p orbitals overlapping.

Double bonds between atoms consist of one sigma bond (from overlapping s orbitals) and one pi bond (from overlapping p orbitals). Triple bonds, though not shown in this molecule, consist of one sigma bond and two pi bonds.

In the molecule shown in this question, we can recognize the following bonds:

1 carbon-oxygen double bond, which is equal to one sigma bond and one pi bond

2 carbon-carbon single bonds, which equates to two sigma bonds

6 carbon-hydrogen single bonds, which equals six sigma bonds

Adding these up, we obtain a total of nine sigma bonds and one pi bond.

This question tests ones knowledge of carboxylic acid derivatives and how different groups effect reactivity. The order is 3, 1, 4, 2 and this is because of the electronic effect. Acid chloride (3) is first because of its electron withdrawing group making it most reactive. (1) is a weakly donating group and (4) is a normal strength donating group and (2) is a good electron donating group. 

The correct answer is D.

The molecule pictured above is an aldehyde because it contains an oxygen double bonded to the last carbon on the carbon chain. A contains a amide group, B contains a ketone, and C contains a carboxylic acid group.

A carbonyl functional group takes the form of . This leaves the carbonyl carbon to form two remaining bonds with various substituents. If one of these substituents is a hydrogen, the molecule is an aldehyde (the carbonyl group is at the end of the carbon chain). If neither substituent is a hydrogen, the molecule is a ketone (the carbonyl group is not at the end of the carbon chain).

An acid contains the carboxylic acid functional group, which takes the form of . Carbonyl groups do not contain the .

Amino acids contain the carboxylic acid functional group () and an amine functional group (). The description in the question does not fit this molecule.

The prefix "form-" indicates a  group. This group by itself is known as the "formate" anion, and its protonated acidic form is known as "formic acid." This is a very common organic acid and it is important to know the name of this acid.

The molecule's longest carbon chain has 4 carbons (thus, "but-"), and the lack of carbon-carbon double bonds makes it an alkANE (thus "butan-"). The presence of a carbonyl () group that is not at the end of the chain means that the molecule is a ketone (thus "butanone"). There is no other way to make a ketone on a carbon chain consisting of 4 carbons, and therefore a locant is not needed when naming this compound.

The molecule's longest carbon chain has 5 carbons (thus, "pent-"), and the lack of carbon-carbon double bonds makes it an alkANE (thus "pentan-"). The presence of a carbonyl () group that is not at the end of the chain means that the molecule is a ketone (thus "pentanone"). Numbering the carbon chain so that the carbonyl is on the lowest possible carbon will put the carbonyl on carbon 2 (Thus "2-pentanone").

The prefix "acet-" or "aceto-" indicates a  group. This group by itself is known as the "acetate" anion, and its protonated acidic form is known as "acetic acid." This is a very common organic acid and it is important to know the name of this acid.

The longest carbon chain is  carbons long (thus ""), and the lack of double bonds makes it an alkane (thus ""). The highest priority functional group is the carboxylic acid group, and the suffix of the IUPAC name should reflect the highest priority functional group (thus ""). Because the carboxylic acid group is assumed to lie on carbon number  there is no need to designate a locand for this functional group. The other substituents of this molecule are the methyl groups on carbons  and  when numbering the chain from left to right. Thus, the name is .