Balance
Fe(s) + Ca(NO3)2(aq)->

Fe(s)+Ca(NO₃)₂(aq)⇒no reaction

In voltaic series  

Li-K-Ba-Ca-Na-Mg-Al-Mn- (H2O) -Zn-Cr-Fe-Cd-Co-Ni-Sn-Pb- (H) -Cu-Hg-Ag-Pt-Au  

The more to the left, the metal is more reactive (easily release electrons) and the stronger reducing agent  

The more to the right, the metal is less reactive (harder to release electrons) and the stronger oxidizing agent  

So that the element located on the left can push the element on the right in the redox reaction  

Reaction

Fe(s)+Ca(NO₃)₂(aq)⇒no reaction

Fe cannot reduce Ca because Ca is more reactive, so the reaction does not occur

On the contrary, this reaction can occur

3Ca(s) + 2Fe(NO₃)₃(aq) = 3Ca(NO₃)₂(aq) + 2Fe(s)

answer by yourhope:

how do models compare to real-world objects?

d. most models represent a real-world object in a simpler way.

: )