Answer:
a. the second nucleotide is wrong. It is a U but should be an A. No other nucleotides were effected by this, only the one was effected. This is a Point (Substitution) Mutation.
b. the third nucleotide is wrong. It is a T that was inserted. As you can see, the rest of the nucleotides are right, but since the T was added, it will mess up how the rest of the sequence is read. It will be read UAT GGC GAA... instead of UAG GCG AAU... see how it messed it up? This is an Insertion (Addition) Mutation.
c. the second nucleotide was deleted. This is called a Deletion Mutation. It will mess up the rest of the sequence, similar to how the example above did.
I hope that helped you understand!