The growth of a pollen tube towards the ovule is caused by....

A phototropism

B. hydrotropism

C. gravitropism

D. chemotropism

The growth of pollen tube towards the ovule is caused in response to a chemical stimulus i.e the chemicals released by the ovule during the time of fertilization.

This is a picture showing chemotropism. Growth of pollen tube Observed in response to chemicals secreted by ovule