Question 1
A pollen grain has landed on a stigma. What stimulates it to begin germinating?
The right answer is the one marked above.
The sugar solution present on the stigma. It does two jobs at once: it is the signal that starts germination, and the food that pays for growing the pollen tube.
The ovule is not reached until the tube arrives, so contact with it cannot be what starts the tube growing — that is contact with the ovule putting the sequence backwards. A grain landing anywhere else gets no sugar, and nothing happens.