= Solution
Expand the <density matrix> of the <Bell state> and take the <partial trace> over Alice's <qubit>:
$$
|\phi_+\rangle\langle\phi_+|=\frac12\sum_{j,k=0}^1|j\rangle\langle k|_A\otimes|j\rangle\langle k|_B,
\qquad
\rho_B=\frac12\sum_{j,k=0}^1\langle k|j\rangle|j\rangle\langle k|.
$$
The <orthonormality> of the <computational basis> removes the off-diagonal terms. Thus Bob's <reduced density matrix> is the <maximally mixed state>
$$
\boxed{\rho_B=\frac12\bigl(|0\rangle\langle0|+|1\rangle\langle1|\bigr)=\frac I2.}
$$
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