Answer
See the detailed answer below.
Work Step by Step
$$\color{blue}{\bf [a]}$$
When the electron beam intensity is increased, there will be no change in the width of the bands but the number of dots increases since each bright dot represents one electron.
$$\color{blue}{\bf [b]}$$
When the electron speed is reduced, the wavelength increases and that means that the fringes will be wider and more spread out, but the number of dots in the same fringe remains constant since the number of electrons remains constant.
$$\color{blue}{\bf [c]}$$
When the electrons are replaced by neutrons, as we know neutrons are heavier than electrons which means more momentum at the same speed. The increase in momentum leads to a smaller wavelength. Hence, the fringes will be narrower and closer together.
$$\color{blue}{\bf [d]}$$
When we close the left slit, (or the right one), we will get the single-slit diffraction pattern and hence the two-slit interference pattern disappears.
