Question

add lone pairs to compound a.

Explanation:

Step1: Determine valence electrons of S

Sulfur has 6 valence electrons.

Step2: Count bonded electrons per S

Each S forms 2 single bonds, so uses 4 electrons.

Step3: Calculate lone pair electrons per S

Lone pair electrons = $6 - 4 = 2$ pairs (4 total electrons).

Step4: Add missing lone pairs

Each S needs 2 lone pairs; the given structure has 1 pair per S, so add 1 more pair to each S.

Answer:

The final structure has each sulfur atom with 2 lone pairs:

     H
     |
H - C - S::S::C - H
     |       |
     H       H

(Each :: represents 2 lone pair electrons, so each S has 4 non-bonding electrons in two lone pairs)