Step 1: Identify the reaction conditions.
The reagent \( (CH_3)_3CO^- \) is tert-butoxide ion, which is a strong and bulky base.
Heat is also provided, which favors elimination over substitution.
Step 2: Determine reaction type.
Due to the bulky nature of tert-butoxide, it prefers elimination (E2 mechanism) rather than substitution.
Thus, dehydrohalogenation occurs, removing \( H \) and \( Cl \) to form an alkene.
Step 3: Identify possible elimination product.
The substrate is \( CH_3-CH(CH_3)-CH_2Cl \).
Elimination of \( H \) from the adjacent carbon and \( Cl \) gives:
\[
CH_2=C(CH_3)-CH_3
\]
Step 4: Reason for product formation.
Although bulky bases often give Hofmann product (less substituted alkene), here only one elimination pathway is feasible leading to the given alkene.
Step 5: Conclusion.
Thus, the major product formed is:
\[
\boxed{CH_2=C(CH_3)-CH_3}
\]