The correct answer is (a) 2n x 1

For explanation: In the vector form, the terms M and K^11 are of the order 2n x 2n, K^12 is of the order 2n x m, K^21 is of the order m x 2n. The equation MΔ+K^11Δ+K^12P=F^1 gives the vector form, where F is force matrix, M is a mass matrix, K^11 is related to velocity term, K^12 is related to the pressure term, P is the pressure matrix, and Δ is the velocity matrix. Note that