A simple example is returning an allocatable array from a function, where the Fortran compiler can decide to allocate on a stack instead, or even inline the function and eliminate completely. While in C the compiler would need to understand the semantics of an allocatable array. If you use raw C pointer and malloc, and use Clang, my understanding is that Clang translates quite directly to LLVM and LLVM is too low level to optimize this out, depending on the details of how you call malloc.
Of course, you can rewrite your C code by hand to generate the same LLVM code from Clang, as LFortran generates for the Fortran code. So in principle I think anything can be done in C, as anything can be done in assembly or machine code. But the advantage of Fortran is that it is higher level, and thus allows you to write code using arrays in a high level way and do not have to do many special things as a programmer, and the compiler can then highly optimize your code. While in C very often you might need to do some of these optimizations by hand as a user.
Of course, you can rewrite your C code by hand to generate the same LLVM code from Clang, as LFortran generates for the Fortran code. So in principle I think anything can be done in C, as anything can be done in assembly or machine code. But the advantage of Fortran is that it is higher level, and thus allows you to write code using arrays in a high level way and do not have to do many special things as a programmer, and the compiler can then highly optimize your code. While in C very often you might need to do some of these optimizations by hand as a user.