**Core Concept**
Rheumatic carditis is a complication of rheumatic fever, an autoimmune response to group A beta-hemolytic streptococcal infection. The inflammation caused by rheumatic fever can affect the heart valves, leading to valvular damage and dysfunction. In the context of valvular manifestations, rheumatic carditis is characterized by inflammation and scarring of the valve leaflets, which can lead to valve stenosis or regurgitation.

**Why the Correct Answer is Right**
Mitral regurgitation (MR) is the commonest valvular manifestation of acute as well as previous rheumatic carditis. This is because the mitral valve is more susceptible to the inflammatory process, particularly the valve leaflets and chordae tendineae. The inflammation and subsequent scarring of the valve leaflets can lead to incomplete closure of the valve, resulting in regurgitation of blood into the left atrium during systole. The mitral valve is also more commonly affected due to its anatomical structure, which makes it more prone to rheumatic damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Mitral stenosis is a valvular manifestation of rheumatic carditis, but it is less common than mitral regurgitation. Mitral stenosis occurs when the valve leaflets become fused together, leading to narrowing of the valve orifice and obstruction of blood flow.
**Option C:** Aortic stenosis is not a common valvular manifestation of rheumatic carditis. While the aortic valve can be affected, the inflammation typically targets the mitral valve.
**Option D:** Aortic regurgitation is also not a common valvular manifestation of rheumatic carditis. The aortic valve is less commonly affected, and when it is, the damage is often more severe, leading to aortic stenosis.

**Clinical Pearl / High-Yield Fact**
The Jones criteria, which are used to diagnose rheumatic fever, include major and minor criteria. The major criteria include carditis, arthritis, chorea, erythema marginatum, and subcutaneous nodules. Mitral regurgitation is a common manifestation of carditis, and its presence can help diagnose rheumatic fever.

**✓ Correct Answer: B. Mitral regurgitation**