According to this dictionary entry, what is the origin of the word skeptic?

skep-tic (skep'tik) [lat. skepticus < gk. skeptikos, thoughtful, inquiring, going back to indoeur. base spek-, to peer, to watch closely] n. 1. [s-] a member of a greek school of philosophers that expressed doubt about generally accepted ideas 2. a person who typically doubts or questions matters that are generally accepted.

a. from the latin word for "to watch closely"
b. from the greek word for "thoughtful"
c. from the indo-european word for "to express doubt"
d. from the greek word for "school