Answer
$(C)~2.5N/m$
Work Step by Step
We can find the required force constant as follows:
$K=\frac{(m_1-m_2)g}{y_2-y_1}$
We plug in the known values to obtain:
$K=\frac{(0.020-0.026)(9.81)}{0.021-0.042}$
$K=2.5N/m$
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