The Federal Trade Commission has sued to block Microsoft from completing its deal to buy video game company Activision Blizzard, the latest antitrust challenge to the proposed merger but one that could hasten its conclusion. The FTC’s Monday filing in a federal court in San Francisco seeks an injunction to stop Microsoft’s $69 billion purchase of the California gaming company behind hit franchises such as Call of Duty and World of Warcraft. Microsoft, maker of the Xbox game system, has been struggling to win worldwide approval for the deal with just over a month before the deadline to close it, according to the contract it signed with Activision.