a. The clerk saw us and them at the counter.
It helps to break the sentences apart.
a. The clerk saw us and the clerk saw them. Both right.
b.Susan knit he sweaters. Wrong
Susan knit I sweaters. Wrong
c. Mom gave she some advice. Wrong
Mom gave I some advice. Wrong
d. The crowd cheered she. Wrong
The crowd cheered me. Right.
Only "a" has all correct object pronouns.