Select the correct option:
Even though the plan looked workable on paper, executing it turned out to be ________ complicated than expected.

The correct option is more.

Explanation:
In English grammar, comparative structures are used to compare the degree of a quality between two situations or things. Here, the sentence compares how complicated the execution turned out to be versus how workable the plan appeared on paper.

The word "even though" introduces a contrast between expectation and reality. Since the plan looked "workable on paper", the contrast implies that executing it was actually harder or more complicated than anticipated.

When forming a comparative adjective for multi-syllable words like "complicated", we use the word more before the adjective (i.e., "more complicated"). Therefore, "more" fits both grammatically and contextually to complete the sentence: "Even though the plan looked workable on paper, executing it turned out to be more complicated than expected."