Question

One function of stems is to physically support leaves in a position to maximize photosynthesis. In large trees, the support function of stems is primarily due to

Answer 1

I Believe it's Xylem.

Answer 2

One function of stems is to physically support leaves in a position to maximize photosynthesis. In large trees, the support function of stems is primarily due to xylem. 
Xylem is the tissue in stems that bring water upwards so that the plant can get what it needs. It's a one way only passage upwards as opposed to the phloem vessel that goes both ways.

Hope this helps.