The correct order of total number of atoms present in
(A) $2$ moles of cyclohexane
(B) $684$ g of sucrose
(C) $90.8$ L of dihydrogen at STP
is:

For (A):

Formula of cyclohexane is $C_6H_{12}$.

Number of atoms per molecule = $6 + 12 = 18$.

Total number of atoms in $2$ moles = $2 \times 18 \times N_A = 36 N_A$.

For (B):

Formula of sucrose is $C_{12}H_{22}O_{11}$.

Molar mass of sucrose = $342$ g/mol.

Number of moles = $\dfrac{684}{342} = 2$ moles.

Number of atoms per molecule = $12 + 22 + 11 = 45$.

Total number of atoms = $2 \times 45 \times N_A = 90 N_A$.

For (C):

Formula of dihydrogen is $H_2$.

Number of moles at STP = $\dfrac{90.8}{22.7} = 4$ moles.

Number of atoms per molecule = $2$.

Total number of atoms = $4 \times 2 \times N_A = 8 N_A$.

Comparing the total number of atoms:

$90 N_A > 36 N_A > 8 N_A$

Therefore, the correct order is $B > A > C$.

Answer: $B>A>C$