Explanation:

- Section 102 of the Evidence Act deals with the burden of proof, specifically stating that the initial burden of introducing evidence lies on the person who would fail if no evidence were given.

- The introduction of evidence, according to this section, does not constantly shift; rather, it is fixed at the beginning. After some evidence is introduced, however, the burden of proof may shift (but that is dealt with in later sections).

- Option 1: "never shifts" — This reflects the correct position under Section 102 for the initial burden of introducing evidence.

- Option 2: "occasionally shifts" — This is not accurate for just the introduction, though the burden of proof may shift during a trial under other provisions.

- Option 3: "constantly shifts" — This is incorrect for Section 102. The initial burden does not keep moving.

- Option 4: "only (a) and not (b) or (c)" — This emphasizes that "never shifts" is the only correct choice here.

The correct answer is Option 1: never shifts.