New sequence is as follows:

pr1  qr2  pr3  pq4  qr5  rp6  qq7  rp8  qp9  qr10  qp11  qp12

16th from the right  end is 7

The size of the pupil of the automatically adjusts itself according to the intensity of light passing through it; if the intensity is more, the size is less, if less, it is more. So in our case the pupil should dilate i.e , increase in size to be able to see in the dark. This switchover takes some time. But here the entry into the dark room is immediate. So the eye has no time to adjust itself and hence is not able to see at all for a while. 

Before transferring the fish, the position is the man with the fish and the bucket of water. After transferring the position is: the man without the fish, but the fish inside the water in the bucket. Thus to know  whether there is any change in weight, it is enough to consider whether there is any change in weight when the fish in air is transferred into the water. The answer is there is no change as when in water though it loses weight due to upthrust of water on it, the level of water increases and exerts an equal force at the bottom.So, (c) is correct.

Fourth to the right of the tenth from the right means (10 - 4 = )6th from the right, ie 5. But according to the queswtion, one is subtracted from each of the numbers, ie 4. 

Specified letters from the left are N and O , and the letter from the right is I.

With these letters , according to the conditions given in the question , one Word ONION can be formed. Clearly , first letter of this word is O. 

It means that the first half of the alphabet is interchanged with second half of the alphabet. After making changes, nth letter from the left will be (n + 13)th letter of original series, ie English alphabet; If n is less than 14. But if n is more than 13, nth letter from left will be (n - 13)th letter from left.
In the given equation, we have to find out (13 - 7 =) 6th letter from right => (26 - 6 + 1 =) 21st letter from the left. Hence, required letter's position is more than 13; therefore the required letter is (21 - 13 =) 8th letter from left of original series, ie H.

8th letter of the left of 14th letter from the right means (8 + 14 =) 22nd letter from the right. 22nd letter from the right means (26 - 22 + 1 =) 5th letter from the left.  We know that 5th letter falls into the first half of the series which is reversed.

Hence, the required letter is (13 - 5 + 1 =) 9th letter from the left of the English alphabet.