To determine which statements about Haryana's folk music are true, let's analyze each statement:

Statement I: 'Bara Masa' is a popular musical form that expresses the emotional journey of separated lovers through the twelve months.

This statement is true. 'Bara Masa', which translates to "twelve months," is a traditional form of music and poetry commonly found in North Indian cultures, including Haryana. It narrates the emotions and struggles of separated lovers as they pass through the changing seasons.

Statement II: 'Jhoolna' songs are associated with religious ceremonies and sung during death rituals.

This statement is false. 'Jhoolna' songs in the context of Haryana are more associated with celebrating swings, especially during the festival of Teej, rather than being linked to death rituals.

Statement III: 'Phag' is a seasonal form of music sung during the Holi festival and expresses joy and playfulness.

This statement is true. 'Phag' is a traditional folk song from Haryana sung during the Holi festival, capturing the spirits of joy, celebration, and playfulness associated with this colorful festival.

Based on the analysis above, the correct statements are I and III. Therefore, the correct answer is:

I and III only