The estimated Ge-H bond length in GeH4 is approximately 1.59 Å, and the weakest X-H bond among CH4, SiH4, GeH4, and SnH4 is expected to be in SnH4.
The estimated length of the Ge-H bond in GeH4 can be inferred from the trend observed in the bond lengths of CH4, SiH4, and SnH4, which increase as we go down the group. Given that Ge is positioned between Si and Sn in the periodic table, it is reasonable to estimate that the Ge-H bond length in GeH4 will be between 1.48 and 1.71 Å. If the increase in bond length is consistent, we can estimate the Ge-H bond to be approximately 1.59 Å, the average of the Si-H and Sn-H bond lengths.
Regarding the weakest X-H bond among the compounds, based on periodic trends, the bond strength decreases as we go down the group in the periodic table, since the larger the atom, the less effective its orbitals overlap with hydrogen, leading to weaker bonds. Therefore, SnH4 would have the weakest X-H bond due to the larger size of the Sn atom compared to C, Si, and Ge.