Đáp án:

(a)

Products (1) and (2) are formed in the substitution reaction. They are each other's optical isomers.

(b)

The tertiary alkyl halide above undergoes a $S_N1$ reaction. The rate-determined step is the formation of an tertiary carbocation, so the nucleophile has equal probability to attack from either side, that leads to a racemic mixture of (1) and (2). The mole ratio of them is 1:1, hence, there is no preference between the two products.

c,

Products (3) to (7) are formed in the elimination reaction. Among them, (4), (6) and (5), (7) are two pairs of stereoisomers in general and geometrical isomers in particular.

d,

Between the two products (4) and (6), product (4) is expected to be favored. Between the two products (5) and (7), product (5) is expected to be favored. Because of the steric hidrance, alkene (4) and (5) are major products, they are more stable than (6) and (7).