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!
A thesaurus maybe, to find the synonyms of convenient.
What the author intended to convey by the statement "ugliness is only skin deep" in line 70 that H. people shouldn't think that bats are harmful simply because they are ugly.
And in fact, I don't even think they are ugly. :)