Explanation

(a)
The reactivity of primary halides is in the order, ${CH}_{3} > CH_{3}CH_{2} > CH_{3}CH_{2}{CH}_{2}$ and the tendency of alkyl halides to undergo elimination is $3{^\circ} > 2{^\circ} > 1{^\circ}$. Hence, for better yield, the alkyl halide should be primary and alkoxide should be secondary or tertiary.
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