The answer is b- Small sample volume.

Refractometers are used to measure the refractive index of a substance, which can be used to determine the concentration of dissolved solids in a liquid. In the case of urine, a refractometer can be used to quickly and accurately determine the specific gravity of the sample, which is a measure of the concentration of dissolved solids in the urine.

On the other hand, a urinometer is a device used to measure the specific gravity of urine by floating a calibrated glass tube in the urine sample. Urinometers require a larger sample volume, and the accuracy of the measurement can be affected by factors such as temperature and the presence of bubbles in the sample.

Therefore, the advantage of using a refractometer over a urinometer is the small sample volume required, which can be especially beneficial in cases where only a small amount of urine is available for testing. Additionally, refractometers are generally considered to be more accurate and reliable than urinometers.

References:

1. Kouri TT, Fogazzi GB. Urine sediment examination: a comparison between the use of the microscope and automated urine analyzers. Clin Chem Lab Med. 2015;53(5):635-648. doi:10.1515/cclm-2014-0642

2. O'Connor OJ, Fitzgerald R, Maher MM. Imaging of urinary tract infection in the adult. Eur Radiol. 2012;22(11):2485-2493. doi:10.1007/s00330-012-2475-8