What does the number 1 in the equation mean in this context?

last option at the bottom.

Step-by-step explanation:

we are told that the the equation

y = 0.73x + 1 represents the Length of the spring when a stretch force is applied,

basically y = length of spring and x = force applied.

from the graph, we can see that when stretch force = 0 (i.e x = 0)

the equation becomes y = 1.

That is to say that the spring has a length of 1 inch when stretch force is zero

(which basically is the same as the last option)