Which of the following is a unit vector?

• A

  $\hat{\mathrm{i}}+\hat{\mathrm{j}}$

• B

  $\cos \theta \hat{\mathbf{i}}-\sin \theta \hat{\mathbf{j}}$

• C

  $\sin \theta \hat{\mathbf{i}}+2\cos \theta \hat{\mathbf{j}}$

• D

  $\frac{1}{\sqrt{3}}(\hat{\mathbf{i}}+\hat{\mathbf{j}})$