Nucleophiles and electrophiles are two types of reagents commonly encountered in organic chemistry. Nucleophiles are electron-rich species that can donate a pair of electrons to form a new bond, while electrophiles are electron-deficient species that can accept a pair of electrons to form a new bond.

In the given options, CH3CN (acetonitrile) behaves both as a nucleophile and an electrophile. Let's analyze why it exhibits both properties.

Nucleophilicity of CH3CN:
1. The carbon atom in CH3CN has a partial positive charge due to the electronegativity difference between carbon and nitrogen.
2. The lone pair of electrons on the nitrogen atom can be donated to another atom or molecule, making CH3CN a nucleophile.
3. This lone pair of electrons can attack an electrophilic center, such as a carbocation or an electrophilic carbon atom, to form a new bond.

Electrophilicity of CH3CN:
1. The carbon atom in CH3CN can also act as an electrophile due to the partial positive charge on it.
2. This partial positive charge makes the carbon atom electron-deficient and susceptible to attack by nucleophiles.
3. Electrophiles can accept a pair of electrons from a nucleophile to form a new bond.

Therefore, CH3CN behaves both as a nucleophile and an electrophile because it can donate its lone pair of electrons to other atoms or molecules as a nucleophile, and it can also accept a pair of electrons from a nucleophile as an electrophile.

In contrast, the other options do not exhibit both nucleophilic and electrophilic behavior:
- CH3OH (methanol) can act as a nucleophile due to the lone pair of electrons on the oxygen atom, but it cannot act as an electrophile.
- CH3NH2 (methylamine) can act as a nucleophile due to the lone pair of electrons on the nitrogen atom, but it cannot act as an electrophile.
- CH3Cl (methyl chloride) can act as an electrophile due to the partial positive charge on the carbon atom, but it cannot act as a nucleophile.

Hence, the correct answer is option C, CH3CN, which exhibits both nucleophilic and electrophilic behavior.