Step 1: Classify fracturing fluids by type: Fracturing fluids used in the field generally fall into a few broad categories, water based slickwater with almost no viscosifier, linear gel with a moderate amount of guar or similar polymer, and cross-linked gel where the linear gel is chemically cross-linked to form a much thicker, more elastic fluid.
Step 2: Rank these categories by viscosity: Slickwater sits at the low viscosity end since only a trace of friction reducer is added, linear gel sits in the middle, and cross-linked gel sits at the high viscosity end because cross-linking bonds the polymer chains into a three dimensional network that traps water and resists flow far more than an uncross-linked solution.
Step 3: Connect viscosity to proppant transport mechanism: Two mechanisms keep proppant suspended in a fracturing fluid, either high fluid viscosity that slows particle settling, or high pump rate turbulence that keeps particles moving. Cross linked gels rely on the first mechanism and can carry very high proppant concentrations even at moderate pump rates, which is exactly why they are chosen for high proppant loading jobs.
Step 4: Compare with the non-gel fluids given in the options: Pentane is used in the industry mainly as an energized or foam component fluid, not as a suspension medium, and its own viscosity is too low to hold proppant. Untreated formation brine is simply produced or native water with dissolved salts and no polymer content, so it behaves hydraulically almost like plain water and cannot suspend proppant either.
Step 5: Reach the conclusion by direct comparison: Among pentane, cross-linked gel, slickwater and untreated brine, only cross-linked gel is engineered specifically with a thick, elastic, high viscosity structure meant to transport and suspend large proppant volumes, making it clearly superior to the other three for proppant carrying capacity.
Final Answer: $oxed{ ext{(B) Cross-linked gel}}$