The colour of ammonium phosphomolybdate is:
The aqueous mixture is heated with concentrated nitric acid and ammonium molybdate. A yellow precipitate of ammonium phosphomolybdate is produced. This yellow precipitate helps us conclude that Phosphorus was present in the organic compound
$Na_3PO_4+3HNO_3\rightarrow H_3PO_4+3NaNO_3$
$H_3PO_4+12(NH_4)_2MoO_4+21HNO_3\rightarrow (NH_4)_3PO_4⋅12MoO_3 (yellow ppt⋅)+21NH_4NO_3+12N_2O$
Hence option B is correct.