Among the options provided, **Casualisation of workforce** is the factor that makes workers highly vulnerable.

1. Casualization of the workforce refers to the practice of employing workers on a temporary or casual basis, often without the job security, benefits, and protections that permanent or salaried employees enjoy. 
2. Workers in casual employment typically face greater job insecurity, uncertainty in income, and are more vulnerable to sudden job loss.

    The other options:

1. **Lack of Employment Skills** can be a barrier to finding stable employment but doesn't necessarily make workers highly vulnerable. With appropriate training and skill development, individuals can improve their employability.
2. **Self-employment** can provide some workers with independence and control over their work, but it also comes with economic risks and may make them vulnerable if their business faces challenges.
3.  **Salaried Employment** generally provides more job security and benefits compared to casual employment, so it is less likely to make workers highly vulnerable.

          Therefore, casualization of the workforce is often associated with high vulnerability among workers.