Why is a change from acg to act a big problem?

Guanine does not pair with Thymine. In a DNA strand, adenine (A) pairs with thymine (T), while guanine (G) pairs with cytosine (C). Therefore, that must mean there was a problem or a mistake with the duplication of the strand.

Because it would result in a different amino acid being coded. Each amino acid in a strand is specified for the protein being built.
When mRNA uses ACG as a template to make amino acids, it will get UGC - which is a Cysteine amino acid.
But if ACT is used as a template, mRNA will get UGA - which is a stop codon. This will make the amino acid strand shorter and would result in a different protein being built.

C. a mature ovule of a plant.