Step 1: Recall the parent ministry first. Ujjwala here is run by the Ministry of Women and Child Development, which immediately signals a welfare and protection scheme rather than a labour or abuse statute.
Step 2: The scheme is built around two pillars. One pillar stops vulnerable women and children from being trafficked, and the other rescues, rehabilitates and reintegrates those who were already exploited for commercial sexual purposes.
Step 3: Comparing that purpose to the listed answers, the option that captures both pillars is child trafficking. The scheme exists precisely to break the cycle of trafficking and to restore its victims.
Step 4: The other choices miss the mark: child abuse and child labour fall under separate frameworks, and None cannot apply when the scheme has an explicit anti-trafficking mandate. So the matched purpose is child trafficking.
\[\boxed{\text{Child trafficking}}\]