Many objective law papers deduct a fraction of a mark for each wrong answer. A deduction of 0.25 changes how you should treat a question you are unsure about.
The arithmetic
With four options and a 0.25 deduction, a blind guess has an expected value of zero: a one-in-four chance of gaining one mark, and a three-in-four chance of losing a quarter. Random guessing neither helps nor hurts on average — but it adds variance.
The rule
If you can eliminate one option with confidence, guessing among the remaining three has a positive expected value. If you can eliminate two, guessing is clearly worth it. If you cannot eliminate any, leave the question blank.
Practise the habit
Practise elimination on any timed MCQ paper you are licensed to use. After each paper, look at the questions you guessed and check whether your eliminations were correct. Over time you will learn how reliable your instincts are.
Always check the marking rule in the official circular for your examination; the deduction may differ.