6   5   4 = 34

Let us check all the options one by one.
From option (a),
6 - 5 + 4 = 34 (using VBODMAS rule)
? 10 - 5 = 34 ? 5 = 34
? LHS ? RHS
So, option (a) is incorrect
From option (b),
6 x 5 + 4 = 34 (using VBODMAS rule)
? 30 + 4 = 34 ? 34 = 34
? LHS = RHS
So, option (b) gives us the correct answer and hence, there is no need to check remaining options.