The median of the first twelve prime numbers is

The first twelve primes are $2,3,5,7,11,13,17,19,23,29,31,37$. With twelve observations the median is the average of the 6th and 7th: $\dfrac{13+17}{2}=15$. Note that $15$ is itself not a prime, so the median need not be an observation. The options $13$ and $17$ pick only one of the middle terms. Listing the primes carefully (and not skipping $2$) avoids shifting the middle positions.