The third sentence.
Using an opinion in an informative piece is always wrong!
I think it is a littleun, the one with the birthmark on their face.
I will correct the mistakes in the sentences:
A. I thought I seen him in the yard.
Mistake! It should be:
A. I thought I saw him in the yard.
C. When had you saw him last?
Mistake! It should be:
C. When had/have you seen him last?
D. Mary sawed that man again yesterday.
Mistake! It should be:
D. Mary saw that man again yesterday.
So, options A, C and D have mistakes, but option D does not have a mistake in it!