This word means the same as the given italicized word. Hence option B is correct.
Option A: 'Irrational' means 'not logical or reasonable'.
Option C: 'Impracticable' means 'impossible in practice to do or carry out'.
Option D: 'Dangerous' means 'able or likely to cause harm or injury'.
These words do not mean the same as the given word 'abstruse'.
So, options A, C and D are wrong.