Step 1: Define work function.
The work function is the least energy needed to pull an electron out of a metal surface. A metal whose electrons are tightly held has a large work function.
Step 2: Recall a useful trend.
Alkali metals such as cesium and sodium release electrons very easily, so their work functions are small. Heavy noble-like metals such as platinum hold electrons tightly, giving large work functions.
Step 3: List approximate values.
$\text{Cs} \approx 2.1\ \text{eV}$, $\text{Na} \approx 2.3\ \text{eV}$, $\text{Al} \approx 4.2\ \text{eV}$, $\text{Pt} \approx 5.6\ \text{eV}$.
Step 4: Eliminate the easy emitters.
Cesium and sodium are the lowest, so they are ruled out for the highest value.
Step 5: Compare the remaining two.
Between aluminium ($\approx 4.2\ \text{eV}$) and platinum ($\approx 5.6\ \text{eV}$), platinum is clearly larger.
Step 6: Conclude.
Hence platinum has the highest work function of the four metals.
\[ \boxed{\text{Platinum (Pt)}} \]