Step 1: Recall what makes fringes.
Interference fringes in Young's experiment need two coherent beams overlapping on the screen.
Step 2: Write the intensity rule.
The combined intensity is $I = I_1 + I_2 + 2\sqrt{I_1 I_2}\cos\phi$, where the cross term creates the bright-dark pattern.
Step 3: Cover one slit.
Blocking one slit removes its light, so $I_2 = 0$.
Step 4: See what survives.
With $I_2 = 0$, the cross term $2\sqrt{I_1 I_2}\cos\phi$ vanishes and only $I = I_1$ remains.
Step 5: Interpret the result.
Without two overlapping beams there is no alternating constructive and destructive pattern; just the broad spread from a single slit reaches the screen.
Step 6: Conclude.
The interference fringes are not observed, so the answer is option (4).
\[ \boxed{\text{Fringes will not be observed}} \]