Assertion : On stimulation, a muscle cell releases calcium ions $\left(\mathrm{Ca}^{2+}\right)$ from sarcoplasmic reticulum. Reason : By reacting with a protein complex, $\mathrm{Ca}^{2+}$ uncover active sites on the actin filaments.

• A

  If both assertion and reason are true and reason is the correct explanation of assertion.
• Correct answer
  B

  If both assertion and reason are true but reason is not the correct explanation of assertion.
• C

  If assertion is true but reason is false.
• D

  If both assertion and reason are false.

Hence, the correct answer is option: 'B'.