In an S_N1 reaction on chiral center, there is:

1. 100% racemization

2. Inversion is more than retention leading to partial racemization. 

3. 100% retention

4. 100% inversion

$\stackrel{\stackrel{\mathrm{H}}{|}}{\mathrm{C}}{\mathrm{H}}_2-\stackrel{\stackrel{\mathrm{Cl}}{|}}{\mathrm{C}}{\mathrm{H}}_2\underset{\mathrm{alk}.}{\overset{\mathrm{NaOH}}{\to }}{\mathrm{CH}}_2={\mathrm{CH}}_2$

Most probable mechanism for this reaction is-

1. E1                                         

2. E2

3. $E{1}_{CB}$                                     

4. $\alpha$ elimination