Step 1: Understand how a computer actually processes instructions.
At the hardware level, a computer is built from digital circuits that can only recognize two electrical states, commonly represented as 0 and 1. Every instruction the processor executes must eventually be reduced to patterns of these two states.
Step 2: Check each option against this hardware reality.
C language, Assembly language and BASIC language are all written using words, symbols or mnemonics that are convenient for a human programmer, but none of these forms can be executed directly by the circuitry.
Step 3: Identify the one language the hardware runs directly.
Machine language consists purely of binary instructions, 0s and 1s, arranged in the exact format the processor is wired to execute, so it is the only option here that the computer understands without any translation step. Assembly, C and BASIC all need an assembler, compiler or interpreter to convert them into machine language first.
\[ \boxed{\text{Machine language}} \]