IN HCHO, C HAS HYBRIDIZATION--------\tSP\tSP2\tSP3\tALL

Since in HCHO C has 3 sigma bond and 0 lone pairs. Therefore, hybridisation state=no. of sigma bond + no. of lone pair i.e., 3+0=0 which is SO2 having trigonal planar like structure

Sorry for mistake instead of that it`s is 3+0=3 i.e., sp3 hybridisation having trigonal planar like structure

Sp2 hybridisation only three sigma bonds present and carbon to oxygen double bond present so total sigma bonds is three

It is formaldehyde and it contains 3 sigma and 1π bond so it`s hybridization will be sp2 and geometry will be trigonal planar