Given below are two statements: one is labeled as Assertion A and the other is labeled as Reason R.

Assertion (A): In a myofibril, H-zone of ‘A’ band is darker than the ‘I’ band.

Reason (R): H-zone contains thick filaments whereas, ‘I’ band contains only thin filaments.

In light of the above statements, choose the most appropriate answer from the options given below.

Assertion (A): In a myofibril, H-zone of 'A' band is darker than the 'I' band.

This statement is correct. The H-zone is a region within the A-band of a sarcomere (the functional unit of a muscle) where only thick filaments (composed mainly of myosin) are present. The arrangement of myosin filaments in the H-zone makes it appear darker under a microscope.

Reason (R): H-zone contains thick filaments whereas, 'I' band contains only thin filaments.

This statement is also correct. The H-zone indeed contains thick filaments, mainly myosin. The I-band is a region in the sarcomere where only thin filaments (primarily composed of actin) are present.

So, the darkness of the H-zone compared to the I-band is due to the presence of thicker myosin filaments in the H-zone. The reason (R) provides an explanation for why the assertion (A) is true.