1. Complex Numbers and Quadratic Equations : This chapter introduces students to complex numbers, which can be a new and abstract concept for many. Understanding operations with complex numbers and solving quadratic equations involving complex roots can be challenging.

2. Limits and Derivatives : Calculus, in general, tends to be difficult for many students, especially when they encounter the concept of limits for the first time. Understanding the concept of limits and their applications, as well as learning differentiation rules and techniques, can be challenging.

3. Permutations and Combinations : This chapter involves various counting principles and problem-solving techniques, which can be confusing for some students. Understanding when to use permutations versus combinations and solving problems involving both can be tricky.

4. Binomial Theorem : The binomial theorem involves expanding expressions raised to positive integer powers, which can require a good understanding of algebraic manipulation and pattern recognition.

5. Statistics : This chapter involves understanding various measures of central tendency and dispersion, as well as probability distributions. It can be challenging for some students to grasp the concepts and apply them to solve problems.

6. Trigonometric Functions : Trigonometry, particularly when it involves proving identities and solving trigonometric equations, can be challenging due to the abstract nature of the concepts and the numerous identities to remember.

Ultimately, the toughest chapter can vary based on individual preferences and strengths. Some students may find certain chapters easier than others based on their prior knowledge or interests in specific topics. Regular practice, seeking clarification when needed, and utilizing resources such as textbooks, online tutorials, and practice problems can help students overcome challenges in any chapter.

Permutation and combination, trigonometry are considered to be considered as difficult chapters by the most of students but with the regular practice students can get command on them also. T rigonometry class 11 ncert solutions help students to get good hold on these topics.