Answer:
C. -35N/m
Step-by-step explanation:
Using Hooke's Law, we know that the elastic constant of a spring can be found with this formula:

where:
is the force ( in Newtons)
is the constant ( in Newtons per meter)
is the extension ( in meters)
thus, by rearranging the formula we can get:



Rounding off the answer, we get -35N/m