In which molecule is the F-A-F angle the smallest if A represents the central atom?

The molecule where the F-A-F angle is the smallest among the options provided is indeed NF3. This is primarily due to the fact that NF3 has a trigonal pyramidal molecular geometry, which results from the presence of a lone pair on the nitrogen atom. The lone pair occupies more space than the bonding pairs, causing the bonding angles (in this case, the F-N-F angle) to be compressed to around 102 degrees, which is smaller than the ideal tetrahedral angle of 109.5 degrees.

In contrast, in F2O, the molecular geometry is bent due to the two lone pairs on the oxygen atom; however, the angles are still more significant than those in NF3 due to the competing repulsion effects of the lone pairs. ClF3 has a T-shaped molecular geometry, and its F-Cl-F angle is constrained by the non-bonding electrons, leading to an even smaller angle that can be significantly below 90 degrees. SF6 has an octahedral geometry, yielding an F-S-F angle of exactly 90 degrees, which is larger than the angles in NF3.

Therefore, NF3 has the smallest F-A-F angle due, in large part, to the influence of its lone pair