Which of the following pair of molecules contain odd electron molecule and an expanded octet molecule?

We need to identify which pair contains one odd-electron molecule and one expanded octet molecule.

An odd-electron molecule is one that has an odd total number of valence electrons, resulting in an unpaired electron. An expanded octet molecule is one where the central atom has more than 8 electrons in its valence shell, which is possible for elements in Period 3 and beyond that can use d-orbitals.

Let us examine each molecule. NO (nitric oxide): Nitrogen has 5 valence electrons, oxygen has 6, giving a total of 11 (odd). So NO is an odd-electron molecule. $$H_2SO_4$$: Sulfur is the central atom bonded to 4 oxygen atoms (two with OH groups). Sulfur has 6 valence electrons and forms bonds with 4 oxygens. In the structure with two S=O double bonds, sulfur has 12 electrons around it — this is an expanded octet. $$BCl_3$$: Boron has 3 valence electrons and forms 3 bonds with Cl, giving only 6 electrons around B — this is an incomplete (electron-deficient) octet, not an expanded octet. $$SF_6$$: Sulfur has 12 electrons around it (6 bonds) — this is also an expanded octet. But $$SF_6$$ is not paired with an odd-electron molecule in the same option as needed.

Looking at Option B: NO (odd-electron molecule) and $$H_2SO_4$$ (expanded octet molecule). This is the correct pairing.

Hence, the correct answer is Option B.