Reaction of methanal with Grignard reagent is as follows :
HCHO+RMgX→RCH2OMgX
H2O
───▶
RCH2OH+Mg(OH)X
From the above reaction, we can see that primary alcohol is produced as a product. Thus, among the given options propan-2-ol is not prepared by the action of Grignard’s reagent on methanal as it is a secondary alcohol.