Answer:
a. of the letters the settlers wrote to relatives back in Spain.
Step-by-step explanation:
San Antonio de Bexar was a historical fort that was built by the Spanish in what is now present-day San Antonio, Texas, US. This place became one of the earliest places of settlement in which the Spanish people came to enter into the American soil, through the process of assimilation and colonial settlement.
In 1675, an expedition consisting of Fray Juan Larios, Fernando del Bosque, Fray Francisco Hidalgo, and Fray Antonio de Olivares was sent by the Spanish crown to look into the possibility of establishing new settlements. It was through this expedition that led to the discovery of San Antonio. Historians were able to learn about the intention of the explorers about making it a permanent settlement from the letters sent to relatives about the need to send more people to the settlement.
Thus, the correct answer is option a.