What form of algae is described as multi-cellular and forming long threads?

The description of multi-cellular algae that form long threads is characteristic of filamentous algae. This type of algae is often seen in aquatic environments and can create visible mats or clumps due to its thread-like structure. Filamentous algae are primarily composed of chains of cells that can stretch over significant lengths, giving them a unique physical form that distinguishes them from other types of algae.

In contrast, planktonic algae are typically single-celled organisms that float freely in the water column, making them unrelated to the multi-cellular and thread-forming characteristics described in the question. Macrophytic algae, often referred to as macroalgae, generally include larger forms such as seaweeds, and may not specifically exhibit the filamentous structure mentioned. Nodular algae, while a valid terminology, is less commonly recognized and does not typically describe the well-known characteristics of filamentous algae.

The specific mention of long strands aligns perfectly with the defining features of filamentous algae, making it the appropriate answer in this context.