Is android kernel a microkernel or a monolithic kernel?
The term ‘monolithic’ is usually applied directly to the kernel of an operating system, rather than the operating system itself which is a boarder, fuzzier concept.

Monolithic kernels, such as Linux, are distinguished from microkernels such as Mach and L4 by having a relatively large amount of code operating in a shared address space. Microkernels tend to have a smaller amount of a shared code and communicate between the components of the kernel using a fast message passing mechanism.

Since the default android kernel is derived from the Linux kernel, it is safe to say that the Android kernel is also a monolithic kernel.