In which of the following compounds, the C-Cl bond ionization shall give most stable carbonium ion?
1) 

2) 

3) 

4) 

Here, bond ionization of C-Cl will give a stable benzyl carbocation due to resonance. We can draw the resonance structures as-
 

As we can see it has 4 resonance structures.

As we can see here that the central carbon atom is attached to only one more carbon atom which means it is a primary carbocation. Therefore, it will not form a stable carbocation.

Here, the carbon is a secondary carbon. Therefore, it is somewhat stable also it has 6 alpha hydrogen atoms therefore it will have 6 hype conjugative structures.
 

• Here, we have a tertiary carbon atom, it will form a tertiary butyl carbocation and it has 9 alpha hydrogen atoms which means it has 9 hype conjugative resonance structures. 
• Therefore, it will also form a stable carbocation.
• As we can see from the above discussion than both tertiary butyl and benzyl carbocation can be the most stable here but in the benzyl carbocation even though the positive charge is delocalized around the benzene ring, it always reacts on the side chain to preserve its aromaticity on the other hand the tertiary butyl carbocation has extra stability due to its planarity (by the weak donation of sigma bond electrons into the empty p-orbital of the cation if at least one C-H bond of each methyl group is parallel to one lobe of the empty p-orbital at one time. 
• Here 3 such donations are possible by the three methyl groups thus making it more stable)
• Planarity is an important factor for stability of carbocation.
•  If it is not stable, it won’t be formed.
• Therefore, the tertiary butyl carbocation will be more stable.