The answer is B.
Given, mean of 25 observations = 36
∴ Sum of 25 observations =36×25=900
Now, the mean of first 13 observations = 32
∴ Sum of first 13 observations =13×32=416
And the mean of last 13 observations = 40
∴ Sum of last 13 observations =40×13=520
So, 13th observation = (Sum of last 13 observations + Sum of first 13 observations) - (Sum of 25 observations)
=(520 + 416) - 900 = 936 - 900 = 36
Hence, the 13th observations is 36.