GTR= General Theory of Relativity
An elevator accelerates upward with an acceleration equal to the acceleration of gravity. A ray of light emitted from one wall inside the elevation at a height H relative to the floor of the elevator will hit the wall at a height ( relative to the floor) less than H or equal to H.
This is a question that need to be explained by making an appeal to the general theory of relativity. In the GTR an acceleration is equivalent to the presence of a mass. You can imagine the problem if there is no elevator acceleration as the beam of light being emitted in the presence of only the Earth mass which attract DOWNWARD the photons. Now if you have an acceleration of the elevator upward this is equivalent to another mass additional to the earth mass that attract UPWARD the photons. (Since the gravitational acceleration is down and the elevator acceleration is UP). Now you have the answer to your question. The beam of light will be deflected UPWARDS relative to the floor relative to the case when there is no acceleration