The nuclei you think is invisible in NMR spectroscopy is:

Explanation

NMR spectroscopy requires nuclei with non-zero spin to be detected. Cl³⁵ has a quadrupolar nucleus with spin 3/2 and often gives broad, weak, or undetectable signals, making it practically "invisible" in routine NMR. Nuclei like C¹³, H¹, N¹⁴, and P³¹ are commonly detectable.