When copper nitrate is strongly heated, it is converted into :
 

When heated in the test tube, copper nitrate breaks down into copper oxide, nitrogen dioxide, and oxygen gas. A water molecule is first lost, and as the process continues to heat up, nitrogen dioxide and oxygen gas are produced.

By dissolving in diluted nitric acid, copper(I) oxide is transformed into copper(II), which may then be thermally degraded into copper(II) oxide.

$2\mathrm{Cu}{\left({\mathrm{NO}}_3\right)}_2\stackrel{\mathrm{\Delta }}{\to }\underset{\text{ Cupric oxide }}{2\mathrm{CuO}}+4{\mathrm{NO}}_2+{\mathrm{O}}_2$

Hence, option b is correct.