To test code against a subset of production data under 5 GB and to refresh the sandbox environment weekly, the most suitable type of sandbox is:

Partial Copy (B). A Partial Copy sandbox includes a copy of your production org's metadata and a subset of your production data as defined by the sandbox template. It allows up to 5 GB of data and can be refreshed every 5 days, making it ideal for scenarios where a representative sample of production data is needed for testing without the storage and refresh limitations of a Full Copy sandbox.

Developer Pro (A) and Developer (D) sandboxes also allow for testing and development but do not include a subset of production data---they either have no data or only limited sample data. A Full Copy sandbox (C) provides a full replica of production data, far exceeding the 5 GB requirement and generally is refreshed less frequently due to larger data volume and longer copy times.

Reference for sandbox types and their capabilities:

Sandbox Types and Templates: https://help.salesforce.com/articleView?id=sf.data_sandbox_implementation_tips.htm&type=5