Incomplete linkage refers to the situation where two genes are located on the same chromosome but do not show complete linkage or do not always assort independently. In a dihybrid cross, two pairs of contrasting traits are considered.

When a dihybrid plant with incomplete linkage is test crossed, it means that it is crossed with a homozygous recessive individual for both traits. This test cross allows us to determine the genotypes of the dihybrid plant by examining the phenotypes of the offspring.

To understand the number of types of plants produced, let's consider an example. Let's say we have a dihybrid plant that is heterozygous for two traits AaBb. It is test crossed with an individual that is homozygous recessive for both traits aabb.

In this case, the two genes A and B are on the same chromosome but do not show complete linkage. This means that recombination can occur between these two genes during crossing over in meiosis.

The possible combinations of alleles that can be produced through recombination are:

- AB: This is the parental combination, where no crossing over occurs.
- ab: This is also the parental combination, where no crossing over occurs.
- Ab: This is a recombinant combination, where crossing over occurs between A and B.
- aB: This is also a recombinant combination, where crossing over occurs between A and B.

So, from the test cross, we can expect four different types of plants to be produced. Each type of plant will have a different combination of alleles for the two traits.

Therefore, the correct answer is option B) 4.