Rate of water flow in soil depends on pressure gradient and:
Hydraulic conductivity (B) is correct here. This is core WATER TRANSPORT: once you know the definition or pathway step, Hydraulic conductivity is the clear fit. The wrong ones are A) Soil texture; C) Root length; D) Temperature. If the topic is about gradients or potentials, water/solutes move from higher to lower of the relevant quantity.
Ref: Best CSIR NET Plant Physiology books: Master Unit 6 with Taiz & Zeiger and Salisbury & Ross. Crack Part C experimental questions with top textbooks.