Answer
See the detailed answer below.
Work Step by Step
$$\color{blue}{\bf [a]}$$
I used my thumb and point finger to measure the diameter of my eye and it is about 3.5 cm.
$$D\approx \color{red}{\bf 3.5}\;\rm cm$$
$$\color{blue}{\bf [b]}$$
The near point distance at which the letters remain sharp is about 14 cm.
The letters here represent the object, so
$$s=\color{red}{\bf 14}\;\rm cm$$
$$\color{blue}{\bf [c]}$$
We know, for thin lenses, that
$$\dfrac{1}{s}+\dfrac{1}{s'}=\dfrac{1}{f}$$
So the effective focal length is
$$f=\left[ \dfrac{1}{s}+\dfrac{1}{s'} \right]^{-1}$$
where we know that the image is formed on the retina, which is located at the back of the eye. Hence, the image distance from the lens front surface is $s'=D$
$$f=\left[ \dfrac{1}{14}+\dfrac{1}{3.5} \right]^{-1}$$
$$f=\color{red}{\bf 2.8}\;\rm cm$$
