The Heuristic method (from the Greek heuriskein, to discover) is based on the principle of learning by doing. Students discover knowledge through their own investigation and experimentation rather than being told the answers. It promotes active, experiential, and self-directed learning.

Correct Answer: A — Learning by doing.

We need $0 < st < 1$, which means $st$ is positive and less than 1.

Check each statement:

I. $s < -1$ and $t > 0$: If $s < -1$ (negative) and $t > 0$ (positive), then $st < 0$ (negative). This violates $st > 0$. ✗

II. $s < -1$ and $t < -1$: Both negative means $st > 0$ ✓. If $s = -2$ and $t = -0.4$, then $st = 0.8$, which satisfies $0 < st < 1$ ✓

Actually, wait. Let's reconsider: if $s < -1$ and $t < -1$, then both are less than $-1$, so both magnitudes are greater than 1. The product would be greater than 1, violating $st < 1$. Let me recalculate: if $s = -1.5$ and $t = -0.5$, then $st = 0.75$, which works. But $t = -0.5$ does not satisfy $t < -1$. If both $s < -1$ and $t < -1$, then $|s| > 1$ and $|t| > 1$, so $|st| > 1$, meaning $st > 1$ since both are negative (making product positive). This violates $st < 1$. ✗

III. $s > -1$ and $t < -1$: This gives us $-1 < s$ and $t < -1$. If $s$ is positive (satisfies $s > -1$) and $t$ is negative with $|t| > 1$, then $st < 0$. ✗ If $s$ is negative with $|s| < 1$ (like $s = -0.5$) and $t < -1$ (like $t = -1.5$), then $st = (-0.5)(-1.5) = 0.75 > 0$ and $< 1$ ✓

Actually, I need to reconsider option I more carefully...

1. Calculate Refractive Index ($\mu$):
$\mu = \frac{c}{v} = \frac{3 \times 10^8}{2 \times 10^8} = 1.5$.

2. Determine Radius of Curvature ($R$):
For a lens with radius of aperture $r = 3 \text{ cm}$ and thickness $t = 0.3 \text{ cm}$, the radius of curvature is given by $R \approx \frac{r^2}{2t}$ (for small thickness).
$R = \frac{3^2}{2 \times 0.3} = \frac{9}{0.6} = 15 \text{ cm}$.

3. Lens Maker's Formula:
$\frac{1}{f} = (\mu - 1) \left( \frac{1}{R_1} - \frac{1}{R_2} \right)$. For a plano-convex lens, $R_1 = R$ and $R_2 = \infty$.
$\frac{1}{f} = (1.5 - 1) \left( \frac{1}{15} \right) = 0.5 \times \frac{1}{15} = \frac{1}{30}$.
Thus, $f = 30 \text{ cm}$.