Based on the information provided:
- Dechen has more marks than David.
- David has more marks than Sonam.
- Karma has more marks than David.
From this, we can establish the following order of marks, from highest to lowest:
- Dechen (highest)
- Karma (more than David)
- David
- Sonam (lowest, because David has more marks than Sonam)
Therefore, Sonam has the lowest marks.