Based on this information, what is this Cepaea snail's phenotype for the shell banding trait?
BB a banded shell
This passage describes the shell banding trait in Cepaea snails:

In a group of Cepaea snails, some individuals have a banded shell and others have an unbanded shell. In this group, the gene for the shell banding trait has two alleles. The allele B is for a banded shell, and the allele b is for an unbanded shell.
A certain Cepaea snail from this group has a banded shell. This snail has two alleles for a banded shell.
All organisms have pieces of hereditary material called genes, which are passed from parents to offspring. Genes contain instructions for building the parts of an organism. An organism's genes affect its observable traits, including its appearance, its behavior, and which diseases it may have. Genes may have different alleles, or forms, that can cause different versions of a trait.
For example, flower color is a trait in pea plants. The gene for this trait has two possible alleles. Each allele is represented by an uppercase or lowercase letter. The allele F is for purple flowers, and the allele f is for white flowers. Each pea plant has two alleles for the flower color gene—one allele inherited from each parent.
An organism's genotype for a gene is its combination of alleles for that gene. So, a pea plant may have a genotype of FF, Ff, or ff for the flower color gene.
An organism's phenotype for a trait is its observable version of that trait, which depends on the organism's combination of alleles. A pea plant may have a phenotype of purple flowers or white flowers for the flower color trait.
An organism's phenotype for a trait is its observable version of that trait. The Cepaea snail's observable version of the shell banding trait is a banded shell. So, the snail's phenotype for the shell banding trait is a banded shell.