Carbonyl compounds are organic compounds that contain a carbonyl group, which is a carbon atom double bonded to an oxygen atom. Isomers are compounds with the same molecular formula but different structural arrangements. In this case, we are looking for carbonyl isomers with a molecular mass of 86 u.

To determine the number of isomers that will give a racemic mixture when treated with CH3MgBr followed by acid hydrolysis, we need to consider the presence of a chiral center in the carbonyl compound.

1. Introduction:
We are given carbonyl isomers with a molecular mass of 86 u.
We need to determine the number of isomers that will give a racemic mixture when treated with CH3MgBr followed by acid hydrolysis.

2. Chiral Center:
A chiral center is an atom in a molecule that is bonded to four different groups. When a chiral center is present, two enantiomers (mirror-image isomers) can form, which would result in a racemic mixture.

3. Calculation:
To calculate the number of isomers that will give a racemic mixture, we need to consider the possible arrangements of atoms around the chiral center.

Since the molecular mass is given as 86 u, we can consider possible combinations of atoms that would result in this mass.

One possible combination is a carbonyl compound with a molecular formula of C5H7O. We can arrange the atoms in different ways to form isomers.

The possible isomers are:
- 2-pentanone (molecular mass = 86 u)
- 3-pentanone (molecular mass = 86 u)
- 4-methyl-2-pentanone (molecular mass = 86 u)
- 2-methyl-3-pentanone (molecular mass = 86 u)

Out of these four isomers, 2-pentanone and 3-pentanone do not have a chiral center, so they will not give a racemic mixture.

However, 4-methyl-2-pentanone and 2-methyl-3-pentanone have a chiral center at the carbon bonded to the carbonyl group. These two isomers will give a racemic mixture when treated with CH3MgBr followed by acid hydrolysis.

Therefore, the correct answer is '4' isomers that will give a racemic mixture.

4. Conclusion:
Out of the four carbonyl isomers with a molecular mass of 86 u, 4-methyl-2-pentanone and 2-methyl-3-pentanone have a chiral center and will give a racemic mixture when treated with CH3MgBr followed by acid hydrolysis. The other two isomers, 2-pentanone and 3-pentanone, do not have a chiral center and will not give a racemic mixture.