What teaching approach does a sped teacher use when varying assignment complexity based on students' ability levels?

The correct choice, which focuses on varying assignment complexity according to students' ability levels, is differentiated instruction. This teaching approach is designed to meet the diverse needs of learners by providing multiple avenues for students to engage with material, show their understanding, and express their learning. By adjusting the complexity of assignments, differentiated instruction allows educators to challenge advanced learners while still providing support and scaffolding for those who may need it, ensuring that all students can progress at their own pace.

In this context, differentiated instruction takes into account various factors such as readiness, interests, and learning profiles, which enables teachers to create an inclusive classroom environment that caters to individual learning needs. This adaptability can manifest in various ways, such as offering tiered assignments that vary in difficulty or providing different resources to accommodate varied learning styles.

Collaborative learning emphasizes group work and peer interaction rather than individual adjustments, while individualized instruction focuses specifically on tailoring education to one student's needs, often without the broader classroom strategies inherent in differentiation. Standardized teaching typically refers to a "one-size-fits-all" approach to instruction that does not address individual student differences. Thus, differentiated instruction is the most effective approach among the options presented for varying assignment complexity based on students' ability levels.