Concept:
When muscles contract in the absence of sufficient oxygen, the body undergoes anaerobic respiration to produce energy. During this process, glucose is broken down incompletely, leading to the formation of lactic acid (lactate).
Accumulation of lactate in muscles may cause muscle fatigue and pain.
Step 1: Understand anaerobic respiration in muscles.
In the absence of oxygen, muscles generate energy through anaerobic metabolism.
Step 2: Identify the substance formed.
The product formed during anaerobic respiration in muscles is:
\[
\text{Lactate (Lactic Acid)}
\]
Hence, the correct answer is:
\[
\boxed{(C)\ \text{Lactate}}
\]