The transition state of SN2 reaction has:  

CONCEPT:

SN2 Reaction and its Transition State

• SN2 (bimolecular nucleophilic substitution) reactions occur in a single concerted step where the nucleophile attacks the electrophilic carbon and displaces the leaving group simultaneously.
• The transition state of an SN2 reaction is characterized by simultaneous bonding of the nucleophile and leaving group to the central carbon atom.
• In this state, the central carbon atom exhibits a unique structure, often referred to as "pentacoordinate" due to five bonds interacting simultaneously.

EXPLANATION:

• The transition state is not a stable structure but a high-energy intermediate that exists momentarily during the reaction.
• In an SN2 reaction:
  • The nucleophile attacks the electrophilic carbon from the opposite side of the leaving group.
  • The structure of the transition state is neither tetrahedral nor planar but has a trigonal bipyramidal geometry.
• Thus, the central carbon atom in the transition state does not have a normal tetrahedral structure or square planar geometry.
• The correct description of the transition state is a carbon with a linear structure due to simultaneous attack and departure of groups.

Therefore, the correct answer is Carbon tetrahedral structure.