The most appropriate tone for an expository essay would be
a. formal.
b. personal.
c. passionate.
d. contemplative.

In an expository essay, you are only asked to describe what your task is, it should be objective and distant, with no poetic vocabulary or what not. So, the correct answer is A, formal, because the other options do not fit here.

1. “very gifted” and “who would succeed”

Explanation:

Albert Schweitzer (1875 – 1965) was a French polymath. He was a citizen of German Empire by birth and later became a French citizen after World War I as his native town Alsac had become part of France. He was a theologian, organist, writer, humanitarian, philosopher, and physician. He received Nobel Peace Prize in 1952 for his philosophy of "Reverence for Life"

Option 1 is correct as it says "very gifted", which means he had gifted talent and "who would succeed in any career" also shows that he had many diverse interests and talent.

Option 2 is not correct because just "thinking of success" has no indication of any talent, "feelings of gratitude" shows humility not talent.

Option 3 is not correct because "develop the idea" is not necessarily related with talent, and "good fortune" too is ambiguous when talking about someone's talent.

Option 4 is not correct because “his own interests” and “serve the world” have no indication of talent.