Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.

[EX Q]: Problem: the mean proportional between 4 and 16 is ?
Options: a. 12, b. 13, c. 14, d. 15, e. 16
[EX A]: e

[EX Q]: Problem: by selling 15 pencils for a rupee a man loses 20 %. how many for a rupee should he sell in order to gain 20 % ?
Options: a. 8, b. 7, c. 6, d. 4, e. 10
[EX A]: e

[EX Q]: Problem: bag a contains red, white and blue marbles such that the red to white marble ratio is 1: 3 and the white to blue marble ratio is 2: 3. bag b contains red and white marbles in the ratio of 1: 4. together, the two bags contain 26 white marbles. how many red marbles could be in bag a ?
Options: a. 1, b. 3, c. 4, d. 2, e. 8
[EX A]:
d