A four bar mechanism is shown below. For the mechanism to be a crank-rocker mechanism, the length of the link PQ can be

The correct option is 80 mm.

Analysis of the Given Four-Bar Mechanism:
Based on the provided schematic diagram (image 0), we can identify the joints and link lengths of the four-bar mechanism as follows:
1. Ground (fixed) link: $PS=400\text{ mm}$
2. Coupler link: $QR=600\text{ mm}$
3. Follower link: $RS=300\text{ mm}$
4. Input link: $PQ$ (with unknown length $l$)

Grashof's Law and Crank-Rocker Conditions:
According to Grashof's law, for a four-bar linkage to have at least one link that can make a complete rotation of $360°$ (making it a Grashof linkage), the sum of the lengths of the shortest ($s$) and longest ($l$) links must be less than or equal to the sum of the lengths of the remaining two links ($p$ and $q$):
$$s+l\le p+q$$
For a linkage to specifically function as a crank-rocker mechanism:
1. The Grashof inequality must be satisfied.
2. The shortest link must be the crank (the link that rotates fully), and it must be adjacent to the fixed link.

Step-by-Step Derivation:
Since the fixed link is $PS=400\text{ mm}$, the adjacent link $PQ$ must be the shortest link ($s=PQ$) to act as the crank.
Since $PQ$ is the shortest link, its length must be smaller than all other link lengths:
$$PQ<300\text{ mm}$$
Under this condition, the longest link is the coupler link, $QR=600\text{ mm}$ ($l=600$).
The other two intermediate links are:
$p=300\text{ mm}$ (link $RS$) and $q=400\text{ mm}$ (link $PS$).

Applying the Grashof inequality:
$$PQ+600\le 300+400$$
$$PQ+600\le 700$$
$$PQ\le 100\text{ mm}$$

Conclusion:
For the mechanism to behave as a crank-rocker, the length of the link $PQ$ must be less than or equal to $100\text{ mm}$.
Among the given options (80 mm, 200 mm, 300 mm, 350 mm), only 80 mm satisfies this criteria ($80\text{ mm}\le 100\text{ mm}$).