The correct answer is option 2: positive phototropism.

Phototropism is the response of a plant to light, causing it to grow or bend towards the light source. In the case of positive phototropism, the plant shoots bend towards the direction of the light source. This occurs because the plant detects the light and produces a hormone called auxin, which is responsible for promoting cell elongation. The cells on the darker side of the shoot elongate faster, causing the shoot to bend towards the light.

Option 1: Photonasty is a general term for movement or response of a plant to light, but it does not specifically refer to the bending of the shoot towards light.

Option 3: Negative phototropism refers to the bending of plant parts away from the light source, which is the opposite of what is described in the question.

Option 4: Photoperiodism refers to the plant`s response to the duration of light and darkness, particularly in relation to flowering and other developmental processes, and is not directly related to the bending of shoots towards light.

In summary, positive phototropism is the correct term to describe the bending of shoots towards light in plants.