Which chromosomal defect is caused when part of a chromosome breaks off and reattaches backward on the same chromosome?
1. inversion
2. translocation
3. deletion
4. insertion

The right answer is inversion.

Inversion is a genetic mutation characterized by the end-to-end reversal of a portion of chromatid on a chromosome.

In other words, it is a double break then it is replaced after rotation of 180 ° of a chromosome segment (backward insertion).

We distinguish :

Paracentric inversions:  The centromere in the chromosome is not included in the inversion.

Pericentric inversions:  The centromere is included in the inversion which transforms a metacentric chromosome into an acrocentric chromosome.

"do a testcross with a homozygous recessive plant."

that is wrong, its not thymus.