What would be the subscripts for the formula unit ca_o_?

CaO    ( So subscripts are 1,1)

Explanation:

In order to determine the possible subscripts of the element/ molecule you need to see the charges that each element has  

In this case  

Ca i.e. calcium has a charge of +2. Charges are present on the element because of the unequal number of electrons. The elements which have a positive charge are called cations

O i.e. Oxygen has a charge of -2 and they are also present due to the uneven number of electrons. The elements which have a negative charge are called anions.

Both the charges +2 of Ca and -2 of O will cancel out and will result in CaO

c (diamond),

sio2 (quartz)