Final answer:
Low blood calcium, or hypocalcemia, causes increased excitability of neurons and involuntary muscle contraction due to increased membrane permeability to sodium.
Step-by-step explanation:
Low blood calcium levels or hypocalcemia can significantly affect the function of neurons and skeletal muscles. In neurons, low blood calcium levels increase membrane permeability to sodium, which leads to increased excitability and the potential for convulsions or muscle spasms. For skeletal muscles, a similar increase in sodium permeability due to low calcium can result in involuntary contractions, or muscle spasms, rather than relaxation.
Therefore, the correct answer to the student's question is c. Increased excitability of neurons; Muscle contraction.