💡 Detailed Explanation
The correct choice is 'ても' (forming '難しくても' - even if it is difficult). The concessive conditional pattern '〜ても' (even if / no matter how...) is used to show that a condition does not change the outcome. Paired with 'どれだけ' (no matter how much / how), '難しくても' is correct. Sentence translation: 'No matter how difficult it is, I will do my best without giving up until the end.' 'たら' (if), 'ば' (if), and 'と' (when/if) are standard conditional forms that do not fit the concessive 'no matter how' structure.