At the end of a reaction it is important to remove the solvent from a solid product (more than one answer may be correct): a. So that the melting point can be determined.
b. So that clean NMR spectra can be obtained that do not contain solvent peaks.
c. So that the yield can be determined.
d. So that the solvent can be reused.

(B.) and (C.)

b. So that clean NMR spectra can be obtained that do not contain solvent peaks.

c. So that the yield can be determined.

Explanation:

The solvent used in Nuclear Magnetic Resonance (NMR) spectrometer is Trimethyl silane (TMS), a neutral solvent which doesn't give off any signals. Other solvents could have interactions with the radiation, and disrupt the spectra.

Furthermore, for accurate determination of the actual yield and overall percentage yield, solid must be separated from the solvent, dried and weighed.

I hope this was explanatory enough.

it means there are 3 atoms of oxygen