fraglich
questionable
Expresses that something is uncertain, doubtful, or not settled.
Es ist fraglich, ob er bei diesem Wetter pünktlich ankommen wird.
It is questionable whether he will arrive on time in this weather.

questionable / in question
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questionable
Expresses that something is uncertain, doubtful, or not settled.
Es ist fraglich, ob er bei diesem Wetter pünktlich ankommen wird.
It is questionable whether he will arrive on time in this weather.

The word 'fraglich' (questionable/in question) is derived directly from the verb 'fragen' (to ask). It describes something that raises a question or can be called into question.
Sein plötzliches Verschwinden ist sehr fraglich.
His sudden disappearance is very questionable.
The meaning of 'fraglich' (questionable/in question) depends on its position in the sentence. If it stands before a noun, it means 'betreffend' (concerning/in question), as in 'der fragliche Tag' (the day in question). If it stands with a verb like 'sein' (to be), it means 'unsicher' (uncertain), as in 'es ist fraglich' (it is questionable).
Der fragliche Brief wurde nie abgeschickt.
The letter in question was never sent.
In the meaning 'unsicher' (uncertain), 'fraglich' (questionable) is very often used in the fixed structure 'Es ist fraglich, ob…' (It is questionable whether…) to express doubt.
Es ist fraglich, ob das neue Gesetz wirklich eine Verbesserung bringt.
It is questionable whether the new law will really bring an improvement.
In the meaning 'betreffend' (concerning/in question), 'fraglich' (in question) is used mainly in formal contexts, such as administrative language, by the police, or in reports.
Die Polizei befragt Zeugen zum fraglichen Zeitpunkt des Unfalls.
The police are questioning witnesses about the time of the accident in question.
sure / safe
'sicher' and 'fraglich' are clear opposites here. 'sicher' means that you have no doubt or that something is very likely. 'fraglich' means that something is still uncertain, doubtful, or not settled. Use 'sicher' when a statement is established or you are strongly convinced of it. 'fraglich' fits when you do not yet know whether something is true, will happen, or is possible.

Wegen des Streiks ist fraglich, ob unser Zug fährt.
Because of the strike, it is questionable whether our train will run.
Morgen ist sicher genug Zeit für das Gespräch.
Tomorrow there will surely be enough time for the conversation.