Definition: In this task, you're given a question, a context passage, and four options which are terms from the passage. After reading a passage, you will get a brief understanding of the terms. Your job is to determine by searching and reading further information of which term you can answer the question. Indicate your choice as 'a', 'b', 'c', or 'd'. If you think more than one option is plausible, choose the more probable option to help you answer the question.
Input: Question: How long did it take Timur to take over Tbilisi? Passage:Timur's first appearance in the Caucasus was a response to Khan Tokhtamysh's marauding inroad into Northern Iran through the Caucasian lands in 1385. This marked an outbreak of outright hostility between the two Islamic monarchs. Timur responded by launching a full-scale invasion of the small frontier countries, which lay between the western border of his emerging empire and Tokhtamysh's khanate. After having overrun Azerbaijan and Kars, Timur marched into Georgia. The official history of his reign, Zafarnama, represents this campaign in Georgia as a jihad. Timur set out from Kars and assailed Samtskhe, the southernmost principality within the Kingdom of Georgia later in 1386. From there, he marched against Tbilisi which the Georgian king Bagrat V had fortified. The city fell on November 21, 1386, and King Bagrat V was captured and converted to Islam at sword point. The Georgian Chronicle and Thomas of Metsoph mention the apostasy of the king but represent it as a clever ruse which enabled him to earn a degree of trust of Timur. Bagrat was given some 12,000 troops to reestablish himself in Georgia whose government was run by Bagrat's son and co-ruler George VII during his father's absence at Timur's court. The old king, however, entered in secret negotiations with George who ambushed Bagrat's Islamic escort, and freed his father.
Links: a. Azerbaijan b. Kars c. Northern Iran d. Siege of Tbilisi (1386)
Output:
d