Aflat, wide, white, fibrous sheet of connective tissue that attaches a flat muscle to a bone or to deep muscles is known as a/an

Aponeurosis

Explanation:

Aponeurosis refers to the connective tissue elements that extend as a broad, flat sheet. Therefore, an aponeurosis refers to a sheet-like tendon that joins one muscle with another or a muscle with bone. Aponeuroses are the extensions of connective tissue that go beyond muscle fibers. They are different from tendons in shape. An aponeurosis is wide and flat. For example, epicranial aponeurosis is present at top of the skull between the frontal and occipital bellies of the occipitofrontalis muscle.

that is wrong, its not thymus.