Which of these elements is the greatest indicator of spring in northern climates in "May-Flower" by Emily Dickinson: the moss, the knoll, the human soul, or the robin?
The short poem known as "May-Flower" by Emily Dickinson is full of compact nature images. When we look at the four you listed, only one of them is most associated with spring. Here are all four images as found in this poem:
Dear to the moss,
Known by the knoll,
Next to the robin
In every human soul.
Moss is used as a symbol, but it is symbolic of time passing (as in the old adage "a rolling stone gathers no moss"). Moss grows slowly and surely, so it is also associated with patience or even decay, both things that require time. Nothing about the symbology of moss seems to be associated with spring.
A knoll is not particularly associated with any season, except perhaps the desolation of a stark winter. A knoll is a raised spot, a small hill or a mound, and it is there in every season. There is nothing about a knoll which associates it with spring.
The human soul is a little bit trickier to assess as a symbol. In fact, usually it is other things which symbolize the soul. The soul has no restrictions or growth based on seasons; in other words, it does not shrivel over the winter and then come alive again in the spring. The human soul is generally not used as a symbol at all.
That leaves the robin. A robin has always been the harbinger (forerunner or precursor) of spring. Visually, the robin's red breast is a picture (or symbol) of dawn, the beginning of a new day. The robin's appearance and song are signals that winter is over and new things have begun. When the songbirds that have gone away for the winter return, spring cannot be far away.
As a symbol of renewal and growth, the robin is clearly the best symbol of spring out of these four choices.