Grappling Hook — ruling 1

If you control the triggered ability, you decide whether to have the targeted creature block the attacking creature at the time the ability resolves, not at the time blockers are declared.

Grappling Hook — ruling 2

If you choose to have the targeted creature block, the creature it must block is the one that was equipped by Grappling Hook at the time the ability triggered. It doesn’t matter if Grappling Hook leaves the battlefield or somehow becomes attached to another creature.

Grappling Hook — ruling 3

If you choose to have the targeted creature block the creature that was equipped with Grappling Hook, but it isn’t able to do so as blockers are declared (for example, because the attacking creature has flying and the targeted creature doesn’t), the requirement to block does nothing. The targeted creature is free to block whichever creature its controller chooses, or block no creatures at all.

Grappling Hook — ruling 4

Tapped creatures, creatures that can’t block as the result of an effect, creatures with unpaid costs to block (such as those from War Cadence), and creatures that aren’t controlled by the defending player are exempt from effects that would require them to block. Such creatures can be targeted by the ability, but the requirement to block does nothing.

Grappling Hook : règles, fonctionnement et interactions (MTG)

Grappling Hook is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Equipped creature has double strike. Whenever equipped creature attacks, you may have target creature block it this turn if able. Equip

How it works : Grappling Hook

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2009· Wizards of the Coast
If you control the triggered ability, you decide whether to have the targeted creature block the attacking creature at the time the ability resolves, not at the time blockers are declared.
No other card is named in this ruling.
#2October 1, 2009· Wizards of the Coast
If you choose to have the targeted creature block, the creature it must block is the one that was equipped by Grappling Hook at the time the ability triggered. It doesn’t matter if Grappling Hook leaves the battlefield or somehow becomes attached to another creature.
No other card is named in this ruling.
#3October 1, 2009· Wizards of the Coast
If you choose to have the targeted creature block the creature that was equipped with Grappling Hook, but it isn’t able to do so as blockers are declared (for example, because the attacking creature has flying and the targeted creature doesn’t), the requirement to block does nothing. The targeted creature is free to block whichever creature its controller chooses, or block no creatures at all.
No other card is named in this ruling.
#4October 1, 2009· Wizards of the Coast
Tapped creatures, creatures that can’t block as the result of an effect, creatures with unpaid costs to block (such as those from War Cadence), and creatures that aren’t controlled by the defending player are exempt from effects that would require them to block. Such creatures can be targeted by the ability, but the requirement to block does nothing.
No other card is named in this ruling.