The tail of a comet is directed away from the sun because

A comet’s tail points away from the Sun because sunlight and the outward-flowing solar wind push material off the comet’s coma. Solar radiation exerts pressure on small dust particles, driving a dust tail away from the Sun, while the corpuscular flow of the solar wind carries ionized gas into a separate, straight plasma tail aligned with the wind’s radial flow. These outward forces overcome the weak binding of the ejected gas and dust produced when the nucleus is heated, so the observable tails are always directed away from the Sun rather than trailing along the comet’s orbital path [3].

Sources

1. [1] https://www.sciencedirect.com/topics/physics-and-astronomy/comet-tails
2. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 2: The Solar System > 2.7. Heliopause — The Boundary? > p. 38
3. [3] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 2: The Solar System > 2. The Solar system > p. 35