(B) 50°
By the tangent-chord angle theorem, ∠CAT = ½ × arc CA, so arc CA subtends 80° at centre. ∠CBA (angle in alternate segment) = 90° − 40° = 50°.
The angle between a tangent and a chord equals the inscribed angle in the alternate segment (Tangent-Chord angle = Alternate Segment Theorem). Here ∠CBA = 90° − ∠CAT = 90° − 40° = 50°. Examiners expect you to identify and apply the alternate segment theorem directly.
Source: Chapter 10, Circles (Tangent to a Circle / properties)