Final answer:
Geographic factors in Texas, specifically the good soil and long growing season, made it viable to develop large, profitable plantations which significantly increased the demand for slave labor, leading to option D being the correct answer.
Step-by-step explanation:
The impact of geographic factors on slavery in Texas comes down to the region's suitability for plantation agriculture. The good soil and a long growing season prevalent in parts of Texas were conducive to the profitable production of cash crops such as cotton. This gave rise to the development of large plantations which, in turn, created a significant demand for the cheap labor that slaves provided. These geographic conditions aligned with economic incentives such as vast tracts of cheap land and tax exemptions offered to settlers and their slaves, underlining the relationship between geography and the institution of slavery in Texas. Thus, option D, which states that good soil and a long growing season made the development of large plantations profitable, best represents how geographic factors in Texas influenced slavery.