The phrase "a challenging topic that" is correct and usable in written English.
You can use it when introducing a subject that requires significant effort or thought to understand or discuss.
Example: "Climate change is a challenging topic that demands our immediate attention and action."
Alternatives: "a difficult subject that" or "a complex issue that".
The simultaneous design and control optimization of intensified distillation sequences is a challenging topic that has partially been addressed by doing several simplifications in the process dynamics.
Multimodal emotion recognition is a challenging topic that aims at determining the affective state of a subject by combining audio-visual and physiological signals acquired in a naturalistic environment.
While I am very sympathetic towards recent natural disasters and issues that have developed in the international sphere, I would advance that gun control is still an issue and a challenging topic that we must taken on.
In this paper, we analytically model the performance of mobile opportunistic networks for contact-based messaging applications in city squares or gathering points, a key challenging topic that is required for the effective design of novel services.
This study has shown that post-neoliberal urban rationalities in time of crises should be understood as a broad and challenging topic that requires more scholarly attention.
The detection of vehicles is an important and challenging topic that is relevant for many applications.
Smart energy big data analytics is also a very complex and challenging topic that share many common issues with the generic big data analytics.
In the new scenario of smart grid, the network complexity will require new methods to overcome new problems that are to come and some that are still a challenging topic.
The design of new coordination polymers is nowadays a challenging research topic that attracts increasing interest due to the unique structural and functional properties of such metal-organic materials.
Our observations suggest that genetic drift is a challenging topic for students to learn.
Nevertheless, as two eminent experts in the field write in a recent overview article, it is still true that "Chinese word segmentation remains a challenging topic in Chinese computational linguistics" (Huang and Xue 2012: 494).
