Kalpana Chawla earned her doctorate in the field of Aerospace Engineering (option B).

Explanation:
Kalpana Chawla was an Indian-American astronaut who achieved great success in her career. She was born on March 17, 1962, in Karnal, Haryana, India. Kalpana Chawla had a deep passion for space exploration and engineering from a young age, which led her to pursue higher education in the field of aerospace engineering.

Here is an explanation of each option provided:

a) Aeronautical Engineering: Aeronautical engineering primarily focuses on the design, development, and maintenance of aircraft. Although closely related to aerospace engineering, it specifically deals with the study of flight within Earth's atmosphere. While aeronautical engineering is an important discipline, it is not the branch in which Kalpana Chawla earned her doctorate.

b) Aerospace Engineering: Aerospace engineering, on the other hand, encompasses both aeronautical and astronautical engineering. It involves the design, development, testing, and operation of aircraft and spacecraft. This branch of engineering is particularly concerned with the study of flight in both Earth's atmosphere and outer space. Kalpana Chawla earned her doctorate in this field, making option B the correct answer.

c) Software Engineering: Software engineering is a branch of computer science that focuses on the development and maintenance of software systems. It involves the application of engineering principles and practices to create efficient and reliable software solutions. This field is not directly related to aerospace engineering, and therefore, Kalpana Chawla did not earn her doctorate in software engineering.

d) Civil Engineering: Civil engineering is a branch of engineering that deals with the design, construction, and maintenance of infrastructure projects such as buildings, bridges, roads, and dams. While civil engineering is an important discipline, it is not the field in which Kalpana Chawla specialized.

In conclusion, Kalpana Chawla earned her doctorate in the field of Aerospace Engineering (option B). Her dedication and achievements in this field have made her an inspiration for aspiring engineers and astronauts around the world.