• Photometric Accuracy & Beer-Lambert Law: In UV-Vis spectrophotometry, photometric accuracy and reproducibility depend on maintaining transmission within the optimal linear dynamic range where the relative error in concentration measurement is minimized.
• Ideal Absorbance Range: The optimal absorbance range for analytical measurement is 0.2 to 0.8 (corresponding to ~15% to 65% transmittance), where spectrophotometric detector noise and stray light errors are minimal. (Correct answer: Option 4)

• HPLC Detection of Simple Sugars:
• Carbohydrates such as D-glucose, D-fructose, and sucrose lack conjugated $\pi$-electron systems or aromatic rings, exhibiting virtually zero UV-visible absorption above $200\text{ nm}$ ($A \approx 0$).
• Consequently, they cannot be monitored with UV/PDA detectors without chemical derivatization.
• In HPLC analysis of carbohydrates, Refractive Index (RI) detection is the universal method of choice because it monitors the differential refractive index between the pure mobile phase and the eluting sugar solute.
• Conclusion: Detected by Refractive Index (RI) detector (Option 2).

Correct option: (2) Refractive index (RI)

• Purity Standard for HPLC Mobile Phase Water:
• According to Good Laboratory Practice (GLP) and ISO/ASTM Type 1 reagent water specifications for HPLC and LC-MS, ultra-pure polished water must have an electrical resistivity of:

• This corresponds to total organic carbon ($TOC$) $\lt 5\text{ ppb}$ and near-zero ionic conductivity, preventing baseline drift and ghost peaks.
• Conclusion: Water should have a resistivity of $18.2\ \text{M}\Omega$ (Option 3).

Correct option: (3) 182M0

• Spectroscopic Structure Elucidation ($C_{10}H_{14}$):
• Degree of unsaturation for $C_{10}H_{14}$:

which indicates a benzene ring ($3\text{ double bonds} + 1\text{ ring}$).

• Two singlets in $^1\text{H}$ NMR:
• 1,2,4,5-tetramethylbenzene (Durene) possesses high $D_{2h}$ symmetry, exhibiting:
• One singlet for the $12$ methyl protons ($\delta \approx 2.2\text{ ppm}$, $4 \times -CH_3$).
• One singlet for the $2$ equivalent aromatic ring protons ($\delta \approx 6.9\text{ ppm}$).
• In $^{13}\text{C}$ NMR, durene gives exactly three signals: substituted aromatic carbon ($ ext{C-CH}_3$), unsubstituted aromatic carbon ($ ext{C-H}$), and methyl carbon ($ ext{-CH}_3$).
• Conclusion: Option 4 represents the correct symmetric isomer (Option 4).

Correct option: (4) Option 4